[#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/put.go

@@ -30,7 +30,7 @@ func (c *cache) Put(o *object.Object) error {
 
 	c.mtx.Lock()
 
-	if sz < c.smallObjectSize && c.curMemSize+sz <= c.maxMemSize {
+	if sz <= c.smallObjectSize && c.curMemSize+sz <= c.maxMemSize {
 		c.curMemSize += sz
 		c.mem = append(c.mem, oi)
 
@@ -43,6 +43,10 @@ func (c *cache) Put(o *object.Object) error {
 
 	c.mtx.Unlock()
 
-	c.persistObjects([]objectInfo{oi})
+	if sz <= c.smallObjectSize {
+		c.persistSmallObjects([]objectInfo{oi})
+	} else {
+		c.persistBigObject(oi)
+	}
 	return nil
 }