Enable auto-commit for session'd requests.

Context: Regular (non-session) requests automatically handle transaction commit/rollback. Session'd requests do not.

As a gremlin server user, I would like the ability to enable automatic transaction handling for session's requests. I would be fine w/ either of the following:

• Configuring my Client to be auto-commit at startup time
• Specifying a "autoCommit: true" arg when submitting a RequestMessage