Move threading concerns to the engine

With the JCA changes we need to more threading concerns primarily to the engine. This move is motivated by transactional connector work needing to execute serially - where the transaction lock is held by the calling thread (a request workitem thread from DQPCore).

We should also forgo the creation of random xids for request and local scoped transactions, rather we can just use the TransactionManager directly like with Teiid 6.2.