mirror: Remove server I/O lock and dirty map

Given our bitset_stream events, we no longer need to worry about
keeping track of the dirty map. This also lets us rip out the
server I/O lock from mirroring.

It's possible that we can remove the lock from client.c as well at
this point, but I need to have a bit more of a think about possible
races

src/mirror.h

@@ -78,7 +78,9 @@ struct mirror {
 	enum mirror_finish_action action_at_finish;
 
 	char                 *mapped;
-	struct bitset_mapping *dirty_map;
+
+	/* Keep an eye on where in the current pass we are */
+	uint64_t pass_offset;
 
 	enum mirror_state    commit_state;