Currently, changefeed relies on laying PTS record during backfills only. After backfill
completion, the record is released.

This behavior has few unfortunate properties:

• A recurring, transient error (e.g unavailable downstream sink) with
  changefeed running with "protect gc on pause" and on_error pause options not be able to protect data if the
  error keeps happening for too long.
• We cannot safely run changefeeds on tables with low gc ttl.
• We cannot safely run changefeeds on (soon to be implemented) ephemeral tables.
• We cannot safely run in serverless environment.

As such, we should change the current PTS management to be much simpler and address
above deficiencies: Namely, PTS record should be created when changefeed starts, and thereafter advanced
with certain periodicity (e.g. checkpoint).