Fix handling of multi-packet queries in readwritesplit

If a prepared statement sends large amounts of data, the target server
where the data is sent will be tracked. The tracked target was not reset
after a multi-packet query was completed and the target itself was used to
check whether the session was processing a multi-packet query.

Changed the check to use the boolean variable instead of the target and
added a reset of the tracked target after a multi-packet query was
completed.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -108,6 +108,11 @@ uint32_t get_internal_ps_id(RWSplitSession* rses, GWBUF* buffer)
     {
         rval = it->second;
     }
+    else
+    {
+        MXS_WARNING("Client requests unknown prepared statement ID '%u' that "
+                    "does not map to an internal ID", id);
+    }
 
     return rval;
 }