Fix for flaky EphemeralFileSystem tests

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.