Better Consistency in Gremlin Server Transaction Management

For sessionless requests to gremlin server, transactions management needs to be tightened such that serialization occurs in the same thread of execution as iteration. This should ensure proper operation across different graphs providers (as some are more strict that others when it comes to transactions). It will also ensure that "extra" transactions are not required to gather data for serialization purposes which should make requests more efficient. Also, transaction management needs to better ensure that the commit of a successful request is complete before the client receives the final streamed message (seems like it was possible for a race condition there)