Track session autocommit state

This tracks only what is explicitly set. That is, if autocommit
has been set true then, even if a transaction is started, autocommit
will not be set false.

That is, a user of the session autocommit and transaction states
need to be aware of their semantics. If a transaction is active,
then the state of autocommit is irrelevant.

server/core/session.c

@@ -143,6 +143,7 @@ session_alloc(SERVICE *service, DCB *client_dcb)
     session->state = SESSION_STATE_READY;
 
     session->trx_state = SESSION_TRX_UNKNOWN;
+    session->autocommit = true;
     /*
      * Only create a router session if we are not the listening
      * DCB or an internal DCB. Creating a router session may create a connection to a