[#922] storage engine: Support operation blocking

There is a need to disable execution of local data operation on storage
engine in runtime. If storage engine ops are blocked, node will act like
always but all local object operations will be denied.

Implement `BlockExecution` / `ResumeExecution` methods on `StorageEngine`
which blocks / resumes the execution of data ops. Wait for the completion of
all operations executed at the time of the call. Return error passed to
`BlockExecution` from all data-related methods until `ResumeExecution` call.
Make `Close` to block operations as well.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/engine/engine.go

@@ -18,6 +18,12 @@ type StorageEngine struct {
 	shards map[string]*shard.Shard
 
 	shardPools map[string]util.WorkerPool
+
+	blockExec struct {
+		mtx sync.RWMutex
+
+		err error
+	}
 }
 
 // Option represents StorageEngine's constructor option.