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/core/gateway.cc

@@ -432,6 +432,7 @@ static void sigfatal_handler(int i)
         if (dcb->session)
         {
             session_dump_statements(dcb->session);
+            session_dump_log(dcb->session);
         }
     }