Synchronizations issues on RrdDbPool #125

RrdDbPool doesn't seem to be really threadsafe :

RrdEntry.count is an integer which is read or written without synchronization, and decisions are taken with the value of this integer. Calling requestRrdDb and release RrdDb could result in unexpected result. Operations on requestRrdDb and releaseRrdDb should probably be synchronized on the RrdEntry

Moreover, the method RrdDbPool.getOpenFiles() could fail with an NPE : it's possible to have on the pool an RrdEntry with a null rrdDb (because it's initializing in another thread by example)