fs: add a runtime debug stream for btrfs tree searches

This allows plugging in an ostream at run time so that we can audit all
the search calls we are doing.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/fs.h

@@ -201,6 +201,7 @@ namespace crucible {
 		static thread_local size_t s_calls;
 		static thread_local size_t s_loops;
 		static thread_local size_t s_loops_empty;
+		static thread_local shared_ptr<ostream> s_debug_ostream;
 	};
 
 	ostream & operator<<(ostream &os, const btrfs_ioctl_search_key &key);