Log Missing Compression-Libraries only as Info because it could be
normal. If somebody wants to use a missed Compression-Library, we will
get a hard exception a bit later, thats why we don't need this
logmessage.
Cleanup Cache on Close.
Integrate Caching of Prepared Statements when they are created per
String.
Don't return defunct connection from the pool.
Revert "Don't return defunct connection from the pool."

This reverts commit 2804dcca90c30ae8c6d3a027c2bf50520b6f2ff1.
Merge remote-tracking branch 'upstream/2.0' into 2.0
Merge remote-tracking branch 'upstream/2.0' into 2.0
Merge remote-tracking branch 'origin/2.0' into 2.1