[#947] writecache: refactor object persisting

a1696a8 introduced some logic which in some situations prevented big objects
to be persisted in FSTree. In this commit a refactoring is done with the
goal of simplifying the code and also checking #866 issue.

1. Split a monstrous function into multiple simple ones: memory objects
   can only be small and for writing through the cache we can do a dispatch
   in `Put` itself.
2. Determine objects to be put in database before the actual update
   as setting up a transaction has non-zero overhead.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/writecache/storage.go

@@ -117,7 +117,6 @@ func (c *cache) deleteFromDB(keys [][]byte) error {
 	if err != nil {
 		return err
 	}
-	c.dbSize.Sub(sz)
 	c.objCounters.DecDB()
 	return nil
 }