#725)

Splitting these changes up into multiple PRs to keep them manageable and
because we can't merge directly to main as `applyChanges` is dependent
on transaction support in the PersistentDataStore.

Existing tests are exercising applyChanges, but through init, upsert,
and delete. Plan to add applyChanges tests in ta/fdv2-temporary-holding
after transaction support is added to PersistentDataStore in a
subsequent PR as part of SDK-1029

**Related issues**

SDK-1029

**Describe the solution you've provided**

Adds TransactionalPersistentStore containing memory store and
persistence store. Swaps on basis being provided as necessary.

Planning to add tests on holding branch (PR target branch).

More tests for FDv2 branch

**Requirements**

- [x] I have added test coverage for new or changed functionality

- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions
Will be done on target temp branch eventually.

**Related issues**

SDK-857

**Requirements**

- [x] I have added test coverage for new or changed functionality

- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions
Will be done on target temp branch eventually.

**Related issues**

SDK-857 and SDK-1073

---------

Co-authored-by: Todd Anderson <tanderson@Todds-MacBook-Pro.local>

**Requirements**

- [x] I have added test coverage for new or changed functionality

- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions
This will be done at a later time on the target branch during
integration testing.

**Related issues**

SDK-858 and SDK-851

**Describe the solution you've provided**

Refactors payload processing to be reusable between polling data sources
and streaming data sources.
Adds OneShotInitializers and PollingSynchronizer.
Adds support to contract tests for sdk-test-harness/feat/fdv2 branch.

---------

Co-authored-by: Todd Anderson <tanderson@Todds-MacBook-Pro.local>

…rable error (#826)

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions
To be done on target branch.

---------

Co-authored-by: Todd Anderson <tanderson@Todds-MacBook-Pro.local>

…er (#832)

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions
Will validate on holding branch later.

**Related issues**

SDK-1179

**Describe the solution you've provided**

Adds FDv1 fallback error type. FDv2 datasources now report that error
type if they see the fallback header from the FDv2 endpoints.

Updates FDv2 polling datasource to support being constructed in a FDv1
to FDv2 conversion mode. In that mode, the response from an FDv1
endpoint will be converted to an FDv2 payload and passed through the
FDv2 payloadProcessor.

CompositeDataSource now supports an FDv1 fallback synchronizer list. If
any datasource reports the FDv1 fallback error, the CompositeDataSource
switches to use the fallback synchronizer list.

This PR also contains tweaks to get the holding branch working with the
latest contract tests that got some tweaks during recent integration
testing.

---------

Co-authored-by: Todd Anderson <tanderson@Todds-MacBook-Pro.local>

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions
Will be done on target branch in the future as part of integration
testing.

**Related issues**

SDK-1044

**Describe the solution you've provided**

Pipes selector through datasource and feature store layers / APIs.
InMemoryFeatureStore holds the selector in memory at runtime. When
datasources need the selector for making their requests, they invoke a
selectorGetter lambda that is hooked up to the InMemoryFeatureStore.

---------

Co-authored-by: Todd Anderson <tanderson@Todds-MacBook-Pro.local>

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [x] I have validated my changes against all supported platform
versions

**Related issues**

SDK-1220

Co-authored-by: Todd Anderson <tanderson@Todds-MacBook-Pro.local>