[#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/delete.go

@@ -16,7 +16,7 @@ import (
 func (c *cache) Delete(addr *addressSDK.Address) error {
 	c.modeMtx.RLock()
 	defer c.modeMtx.RUnlock()
-	if c.mode == ModeReadOnly {
+	if c.readOnly() {
 		return ErrReadOnly
 	}