feat(replication): add Single Active Replication per policy
* Added single_active_replication field to schema & DB table
* Updated API, controller & handler to enforce single execution per policy
* Added checkbox in UI to enable/disable single_active_replication for a
policy
* Implemented necessary backend & frontend tests
* Prevents parallel runs per policy if single_active_replication is enabled
Signed-off-by: bupd <bupdprasanth@gmail.com>