[everything] Fix circular shared pointer references.

We had a cycle from transaction_manager <-> space_map, and also from
the ref_counters back up to the tm.

This prevented objects being destroyed when various programs exited.

From now on we'll try and only use a shared ptr if ownership is
implied.  Otherwise a reference will be used (eg, for up pointers).

thin-provisioning/restore_emitter.cc

@@ -134,7 +134,7 @@ namespace {
 	private:
 		single_mapping_tree::ptr new_mapping_tree() {
 			return single_mapping_tree::ptr(
-				new single_mapping_tree(md_->tm_,
+				new single_mapping_tree(*md_->tm_,
 							mapping_tree_detail::block_time_ref_counter(md_->data_sm_)));
 		}