[PATCH] Enable application-level management of IndexWriter.ramDirectory size

IndexWriter currently only supports bounding of in the in-memory index cache using maxBufferedDocs, which limits it to a fixed number of documents. When document sizes vary substantially, especially when documents cannot be truncated, this leads either to inefficiencies from a too-small value or OutOfMemoryErrors from a too large value.

This simple patch exposes IndexWriter.flushRamSegments(), and provides access to size information about IndexWriter.ramDirectory so that an application can manage this based on total number of bytes consumed by the in-memory cache, thereby allow a larger number of smaller documents or a smaller number of larger documents. This can lead to much better performance while elimianting the possibility of OutOfMemoryErrors.

The actual job of managing to a size constraint, or any other constraint, is left up the applicatation.

The addition of synchronized to flushRamSegments() is only for safety of an external call. It has no significant effect on internal calls since they all come from a sychronized caller.