Modified DynamicByteBuffer in EphemeralFileSystem such that it no longer clears itself out after
a database is shut down, but lives on as a JVM-wide pool of byte buffers until the JVM is shut down,
or until the JVM needs the memory and garbage collects the pools.

This addresses a race condition where one database being shut down would clear the pools,
leaving a brief window where another database could get null back when asking for a pool, causing
crashes.
Merge pull request #484 from jakewins/dynamicbytebuf-pools

Fix for flaky EphemeralFileSystem tests