Add exit_connect, start_server, stop_server hooks.

These would be similar to the init_connect command that is executed when a
connection is created.

The exit_connect command would be executed as the connection user.
The start_server and stop_server should be run as user 'root' with SUPER_ACL
privileges)
(Note that this is troublesome as the server may not have a 'root' user, so we
may want to allow the user to specify that name of the super user on the command
line)

Observers changed: Sergei