context: keep the resolve cache smaller

We don't need to cache 65536 extent maps, especially if each one
can have almost 700K references.

Valgrind's massif tool points to the extent map cache as a very
large memory allocator, but test runs with memcg disagree.

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

src/bees.h

@@ -766,6 +766,7 @@ public:
 
 	BeesResolveAddrResult resolve_addr(BeesAddress addr);
 	void invalidate_addr(BeesAddress addr);
+	void resolve_cache_clear();
 
 	void dump_status();
 	void show_progress();