MXS-1825: Fix PS output parameter tracking for MySQL variants

The resultset processing for MySQL requires some extra work as it lacks
the proper SERVER_MORE_RESULTS_EXIST flag in the last EOF packet. Instead,
the first EOF packet has the SERVER_PS_OUT_PARAMS flag which needs to be
interpreted as a SERVER_MORE_RESULTS_EXIST flag for the second EOF packet.

Also corrected the EOF packet handling to do the flag checks in the code
that deals with the EOF packets.

As the modutil_state parameter is now used for more than large packet
tracking, the correct solution is to store this state object in the
readwritesplit session instead of interpreting it to a boolean value.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -17,7 +17,7 @@
 RWBackend::RWBackend(SERVER_REF* ref):
     mxs::Backend(ref),
     m_reply_state(REPLY_STATE_DONE),
-    m_large_packet(false),
+    m_modutil_state({}),
     m_command(0),
     m_opening_cursor(false),
     m_expected_rows(0)