Re: Single transaction in the tablesync worker?

Hi Amit.

PSA my v8 WIP patch for the Solution1.

This has the same code changes as the v7 patch, but the v8 patch can
be applied to the current PG OSS master code base.

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Coded / WIP:

* tablesync slot is now permanent instead of temporary. The tablesync
slot name is no longer tied to the Subscription slot name.

* the tablesync slot cleanup (drop) code is added for DropSubscription
and for finish_sync_worker functions

* tablesync worked now allowing multiple tx instead of single tx

* a new state (SUBREL_STATE_COPYDONE) is persisted after a successful
copy_table in LogicalRepSyncTableStart.

* if a relaunched tablesync finds the state is SUBREL_STATE_COPYDONE
then it will bypass the initial copy_table phase.

* tablesync sets up replication origin tracking in
LogicalRepSyncTableStart (similar as done for the apply worker). The
origin is advanced when first created.

* tablesync replication origin tracking is cleaned up during
DropSubscription and/or process_syncing_tables_for_apply

* The DropSubscription cleanup code was changed lots in v7. The
subscription TAP tests have been executed 6x now without observing any
race problems that were sometimes seen to happen in the v6 patch.

TODO / Known Issues:

* Help / comments

* There is temporary "!!>>" excessive logging scattered around which I
added to help my testing during development

* Address review comments

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Kind Regards,
Peter Smith.
Fujitsu Australia