Implement logical replication (transform, postgres)

[nevella]

For replica set [primary, secondary], appServers [servers]

• (manual) modify secondary
  • stop secondary
  • do xxx
  • restart secondary [as writeable, not hot standby]['new primary']
  • start non-public app server (catchup app server) with property "logicalReplicationPostgresUrl"=
• pre-catchup configuration
  • mark catchup app server as 'do not proxy to', 'do not participate in leadership'
• catchup
  • loop on unprocessed dtrs [old primary]
  • get batch (say 100)
  • if batch size < rq size, close to finished: signal non catchup servers to pause transform processing outside db transactions
  • apply requests (with possible translation) to [new primary]
  • if batch size = 0, no new requests for 5 secs and all non catchup servers indicate no in-db-tx processing , exit loop
• post-catchup configuration
  • switch non-catchup server postgres urls
  • mark catchup app server as 'proxy to', 'participate in leadership'