Add estimated lag metric for CDC based replication in MIRROR UI

[saisrirampur]

Estimated lag can be calculated by running the below query on source Postgres database:

SELECT slot_name,

round((redo_lsn-restart_lsn) / 1024 / 1024 / 1024, 2) AS GB_behind

FROM pg_control_checkpoint(), pg_replication_slots WHERE slot_name='slot_name_associated_to_that_mirror';

[Amogh-Bharadwaj]

Closing as lag is shown in our Postgres peer page: #576