Fix EOF packet calculation for large rows

The EOF packet calculation function in modutil.cc didn't handle the case
where the payload exceeded maximum packet size and could mistake binary
data for a ERR packet.

The state of a multi-packet payload is now exposed by the
modutil_count_signal_packets function. This allows proper handling of
large multi-packet payloads.

Added minor improvements to mxs1110_16mb to handle testing of this change.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -16,7 +16,8 @@
 
 RWBackend::RWBackend(SERVER_REF* ref):
     mxs::Backend(ref),
-    m_reply_state(REPLY_STATE_DONE)
+    m_reply_state(REPLY_STATE_DONE),
+    m_skip(false)
 {
 }
 
@@ -34,6 +35,16 @@ void RWBackend::set_reply_state(reply_state_t state)
     m_reply_state = state;
 }
 
+void RWBackend::set_skip_packet(bool state)
+{
+    m_skip = state;
+}
+
+bool RWBackend::get_skip_packet() const
+{
+    return m_skip;
+}
+
 bool RWBackend::execute_session_command()
 {
     bool expect_response = mxs_mysql_command_will_respond(next_session_command()->get_command());