[#1143] shard: Introduce explicit Degraded mode

`Degraded` mode is set automatically after error counter is over the
threshold. `ReadOnly` mode can still be set by an administrator.

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

pkg/local_object_storage/writecache/persist.go

@@ -21,7 +21,7 @@ func (c *cache) persistLoop() {
 		select {
 		case <-tick.C:
 			c.modeMtx.RLock()
-			if c.mode == ModeReadOnly {
+			if c.readOnly() {
 				c.modeMtx.RUnlock()
 				continue
 			}