[FTS] Just changing number of replicas shouldn't rebuild all pindexes

Changing the number of replcias causes all pindexes to be rebuilt from scratch, even if no other changes are made to the index definition. This is because the number or replicas is set as part of the index definition and it is treated like any other index definition change, and other changes to the index definition really do need to reindex the data.

If a user only changes the number of replicas and nothing else in the index definition, rebuilding the indexes from scratch causes the system to do more work than necessary and will not be what the user expects to have happen. This may also reduce index availability.