storage: use map size hints to optimize subsequent persist()

We're likely to have something comparable to the current changeset in the
subsequent one. If it's bigger, no big deal, it'll be reallocated, if it's
smaller, no big deal, the next one will be preallocated smaller.

pkg/core/storage/memcached_store.go

@@ -252,8 +252,8 @@ func (s *MemCachedStore) persist(isSync bool) (int, error) {
 	// unprotected while writes are handled by s proper.
 	var tempstore = &MemCachedStore{MemoryStore: MemoryStore{mem: s.mem, del: s.del}, ps: s.ps}
 	s.ps = tempstore
-	s.mem = make(map[string][]byte)
-	s.del = make(map[string]bool)
+	s.mem = make(map[string][]byte, len(s.mem))
+	s.del = make(map[string]bool, len(s.del))
 	if !isSync {
 		s.mut.Unlock()
 	}