[#1806] writecache: Allow to start flush manually

Allow user to initiate flushing objects from a writecache.
We need this in 2 cases:
1. During writecache storage schema update, it should be flushed with
   the old version of node and started clean with a new one.
2. During SSD replacement, to avoid data loss.

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

pkg/local_object_storage/writecache/writecache.go

@@ -28,6 +28,7 @@ type Cache interface {
 	SetMode(mode.Mode) error
 	SetLogger(*zap.Logger)
 	DumpInfo() Info
+	Flush() error
 
 	Init() error
 	Open(readOnly bool) error