MXS-3273: Ignore unexpected responses

Unlike readwritesplit, schemarouter will process all responses from
backends as if they are expected. There are cases where errors are
generated that aren't sent as a response to a query. These queries must be
ignored and not routed to the client. Copying the code as-is from
readwritesplit isn't the cleanest solution but it avoids refactoring code
in a patch release.

The custom error number (2003) used by the backend protocol code was not
an actual error number that the server would send. The error code in
question was for an error that only the C connector returns:
CR_CONN_HOST_ERROR. Using ER_CONNECTION_KILLED as the error number better
conveys the fact that the connection was killed due to a reason not
related to any ongoing query.

By using a known error number that is correctly handled, we also avoid
writing errors to the client in the middle of a resultset or as the
initial response to a result. This explains why the problem described in
MXS-3267 happened in the first place: an unrelated connection was lost in
the middle of a resultset and the error was interpreted as the end of a
resultset. As a result of there being more data to be read, the unexpected
result state messages were logged.

server/modules/protocol/MySQL/mysql_common.cc

@@ -207,7 +207,7 @@ GWBUF* mysql_create_custom_error(int packet_number,
     mysql_state = "HY000";
 
     field_count = 0xff;
-    gw_mysql_set_byte2(mysql_err,    /* mysql_errno */ 2003);
+    gw_mysql_set_byte2(mysql_err, ER_CONNECTION_KILLED);
     mysql_statemsg[0] = '#';
     memcpy(mysql_statemsg + 1, mysql_state, 5);