s3: Avoid starving locks when many processes die at the same time

s3: Avoid starving locks when many processes die at the same time

In g_lock_unlock we have a little race between the process_exists and
messaging_send call: We only send to 5 waiters now, they all might have died
between us checking their existence and sending the message. This change makes
g_lock_lock retry at least once every minute.