Transaction cache cannot be disabled

Tested on JanusGraph-HBase 0.2.0

One would expect the transaction cache to be disabled when "storage.transactions=false" is set, but this is not the case. Disabling can be useful while doing performance tests. It is not a big deal because a simple workaround is to issue a g.tx().close() after each query.

Related to this, it is not documented that the transaction cache size is 100 at minimum, so the following does not work either for disabling the transaction cache:
cache.tx-cache-size = 0
cache.tx-dirty-size = 0

Start by tracing how the storage.transactions setting and cache.tx-cache-size/cache.tx-dirty-size options are handled in the transaction cache configuration. Verify the documented minimum cache size and the behavior when storage.transactions=false, then add or run tests showing that the cache can be disabled and that the configuration is documented accurately.

Written by the indexing model from the issue text.