Initial Thrift connection pool changes

This is the first commit in this refactoring branch that compiles.  It
is untested.  The titan-cassandra test suite passed on my box except
for these failures:

testCreateDelete(com.thinkaurelius.titan.graphdb.astyanax.InternalAstyanaxGraphTest): expected:<353> but was:<null>

testThreadBoundTx(com.thinkaurelius.titan.graphdb.astyanax.InternalAstyanaxGraphTest): expected:<65> but was:<null>

deletionsAppliedBeforeAdditions(com.thinkaurelius.titan.diskstorage.cassandra.astyanax.InternalAstyanaxMultiWriteKeyColumnValueTest): expected:<java.nio.HeapByteBuffer[pos=0 lim=1 cap=1]> but was:<null>