Use consistent tx for schema retrieval

Experimenting with opening a new transaction with a reference held on
the stack solely for the purpose of reading schema vertices and schema
relations.

For #881
Disable cache on correct tx for schema reads

de177b9e59b02cd865fb9e49aa4d1a120353ced8 modified
StandardTitanGraph.typeCacheRetrieval's retrieveSchemaByName and
retrieveSchemaRelations methods to each instantiate a new
StandardTitanTx using a key-consistent backend config (i.e. Cassandra
QUORUM) inside each retrieve method to perform schema reads.  Those
methods also disabled cache on a transaction just before the schema
reads.  However, they disabled cache on the wrong tx -- when I
introduced a a new key-consistent tx to these methods for schema
reads, I failed to move the disableCache invocation from the old tx to
the new key-consistent one.

Now cache is disabled on the actual StandardTitanTx used for the
schema schema vertex/relation read

For #881
Removing tx from SchemaCache method params

Now that the SchemaCache.StoreRetrieval implementation constructs a
new, cache-disabled transaction for each schema read method
invocation, it no longer uses the tx param to its various methods.
This lets us remove the tx param from both
SchemaCache.StoreRetrieval's methods and SchemaCache's methods.

For #881
Add helper method for exception/null-safe rollback

This commit introduces a TXUtils.rollbackQuietly which is roughly
similar to IOUtils.closeQuietly.  This might become obsolete in Titan
0.9 where we can assume try-with-resources and txs are AutoCloseable.

First used as part of #881's implementation, but not specifically
related to that issue
Tweak TitanOperationCountingTest expectations

This test was broken by de177b9e59b02cd865fb9e49aa4d1a120353ced8 and
03a4c5fb8919d2ff5c0a00c705f0a43818751a95.  Updating it to reflect the
changes to the way Titan reads schema elements following those
changes.