[#2057] meta: Fix concurrent mode changes

Includes:
1. mode change read lock operation in every exported method that r/w the
underlying database;
2. returning `ErrDegradedMode` logical error if any exported method is
called in degraded (without a metabase) mode.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/local_object_storage/metabase/exists.go

@@ -44,6 +44,10 @@ func (db *DB) Exists(prm ExistsPrm) (res ExistsRes, err error) {
 	db.modeMtx.RLock()
 	defer db.modeMtx.RUnlock()
 
+	if db.mode.NoMetabase() {
+		return res, ErrDegradedMode
+	}
+
 	currEpoch := db.epochState.CurrentEpoch()
 
 	err = db.boltDB.View(func(tx *bbolt.Tx) error {