Have server status updates applied during monitor loop

Previously, server status changes from MaxAdmin would be set immediately
as long as the server lock could be acquired. This meant that it might take
several seconds until the next monitor pass is executed. Usually, this was
fine but in some situations we would want the monitor to run immediately
after the change (MXS-740 and Galera). This patch changes the logic of
setting and clearing status bits to a delayed mode: changes are first applied
to a "status_pending"-variable, and only once the monitor runs will the
setting be applied. To reduce the delay, the monitor now has a flag
which is checked during sleep (between short 0.1s naps). If set, the
sleep is cut short.

If a server is not monitored, the status bits are set directly.

There is a small possibility of a race condition: If a monitor is stopped or
destroyed before the pending change is applied, the change is forgotten.

include/maxscale/server.h

@@ -96,6 +96,7 @@ typedef struct server
     char           *auth_options;  /**< Authenticator options */
     SSL_LISTENER   *server_ssl;    /**< SSL data structure for server, if any */
     unsigned int   status;         /**< Status flag bitmap for the server */
+    unsigned int   status_pending; /**< Pending status flag bitmap for the server */
     char           monuser[MAX_SERVER_MONUSER_LEN]; /**< User name to use to monitor the db */
     char           monpw[MAX_SERVER_MONPW_LEN]; /**< Password to use to monitor the db */
     SERVER_STATS   stats;          /**< The server statistics */