Changed default setting for keeping forward index in memory to false.

It's generally better to rely on OS caching instead.
Added a simple resource pool class.(SimpleResourcePool)

To be used for ensuring thread-safeness.
Made sure total number of hits is only counted when requested.

This saves time on large queries where we're not interested in the
total.
Added test utilities for separately batch testing parts of BlackLab.

Each utlilty fires off a batch of operations on either the Lucene index,
the forward index or the content store.
Made ContentStoreDirZip thread-safe.

Now uses a resource pool for the Deflater and Inflater objects.