MXS-2592 Add configuration for session specific in-memory log

When enabled each session will write log messages in the in-memory log.
If session ends in error this log is written to the actual log in disk.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -395,6 +395,7 @@ static void log_unexpected_response(SRWBackend& backend, GWBUF* buffer, GWBUF* c
                   backend->current_command(),
                   sql.c_str());
         session_dump_statements(backend->dcb()->session);
+        session_dump_log(backend->dcb()->session);
         mxb_assert(false);
     }
 }