Add `_conversationAdded` and `_conversationRemoved` hooks.

Support _rtmClientSign hook.

creating conversations supports `is_unique`

Features:

- Support Android Key (`LEANCLOUD_APP_ANDX_KEY`) in LeanEngine environment.
- push.send now supports new parameter `flow_control` for smooth push.

Merge pull request leancloud#424 from weakish/release-2.2.0

docs: release notes of 2.2.0

🔖 release 2.1.14

🔖 release 2.1.13

Fixed

- Push: expiration_time
- Push: updated_keys
- LeanEngine: X-LC-Prod request header

Merge pull request leancloud#401 from leancloud/develop/smiao

2.1.11

🔖 bump version