Make ConsistentLookup implement IsBackfilling

Signed-off-by: Armand Parajon <[email protected]>

go/vt/vtgate/vindexes/consistent_lookup.go

@@ -489,7 +489,7 @@ func (lu *clCommon) GetCommitOrder() vtgatepb.CommitOrder {
 }
 
 // IsBackfilling implements the LookupBackfill interface
-func (lu *ConsistentLookupUnique) IsBackfilling() bool {
+func (lu *clCommon) IsBackfilling() bool {
 	return lu.writeOnly
 }