MXS-1780 Collect server response information

As the router is the only one that knows what backends a particular
statement has been sent to, it is the responsibility of the router
to keep the session bookkeeping up to date. If it doesn't we will
know what statements a session has received (provided at least some
component in the routing chain has RCAP_TYPE_STMT_INPUT capability),
but not how long their processing took. Currently only readwritesplit
does that.

All queries are stored and not just COM_QUERY as that makes the
overall bookkeeping simpler; at clientReply() time we do not need to
know whether or not to bookkeep information, we can just do it.

When session information is queried for, we report as much information
we have available.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -585,6 +585,9 @@ void RWSplitSession::clientReply(GWBUF* writebuf, DCB* backend_dcb)
     {
         /** Got a complete reply, decrement expected response count */
         m_expected_responses--;
+
+        session_book_server_response(m_pSession, backend->backend()->server, m_expected_responses == 0);
+
         mxb_assert(m_expected_responses >= 0);
         mxb_assert(backend->get_reply_state() == REPLY_STATE_DONE);
         MXS_INFO("Reply complete, last reply from %s", backend->name());
@@ -639,6 +642,7 @@ void RWSplitSession::clientReply(GWBUF* writebuf, DCB* backend_dcb)
             m_otrx_state = OTRX_INACTIVE;
             start_trx_replay();
             gwbuf_free(writebuf);
+            session_reset_server_bookkeeping(m_pSession);
             return;
         }
     }