metamorphic: sync on derived dbs in ingestOp

Ingesting a batch closes it. As a result we should also be syncing
on the db where a batch came from, even if it's not the db that the
batch was ingested into. This prevents cases where we close the db
before we ingest a batch that came off of it, and deletes on the
batch would then panic because the underlying db (where we do an
isFMV() call) would be nil by then.

Fixes #3055.

metamorphic/ops.go

@@ -625,7 +625,17 @@ func (o *ingestOp) receiver() objID { return o.dbID }
 func (o *ingestOp) syncObjs() objIDSlice {
 	// Ingest should not be concurrent with mutating the batches that will be
 	// ingested as sstables.
-	return o.batchIDs
+	objs := make([]objID, 0, len(o.batchIDs)+1)
+	objs = append(objs, o.batchIDs...)
+	addedDBs := make(map[objID]struct{})
+	for i := range o.derivedDBIDs {
+		_, ok := addedDBs[o.derivedDBIDs[i]]
+		if !ok && o.derivedDBIDs[i] != o.dbID {
+			objs = append(objs, o.derivedDBIDs[i])
+			addedDBs[o.derivedDBIDs[i]] = struct{}{}
+		}
+	}
+	return objs
 }
 
 func closeIters(