Extract ReaderPool from IndexWriter

ReaderPool plays a central role in the IndexWriter pooling NRT readers and making sure we write buffered deletes and updates to disk. This class used to be a non-static inner class accessing many aspects including locks from the IndexWriter itself. This change moves the class outside of IW and defines it's responsiblity in a clear way with respect to locks etc. Now IndexWriter doesn't need to share ReaderPool anymore and reacts on writes done inside the pool by checkpointing internally. This also removes acquiring the IW lock inside the reader pool which makes reasoning about concurrency difficult.

This change also add javadocs and dedicated tests for the ReaderPool class.

/cc mikemccand dawidweiss