Fix memory leaks in LRUVertexCache and StandardTitanTx

The recently introduced LRUVertexCache slowly consumes tenured heap over time.  Transaction commit and rollback were not completely releasing cache resources used by the transaction during its lifetime.

I believe it is wise to build the vertex cache with softValues() along with a maximum capacity.  This way, applications can recover from misjudgments in setting the transaction cache capacity rather than throwing excessive GC overhead exceptions.