Tablesync early exit

Hi hackers.

I propose a small optimization can be added to the tablesync replication code.

This proposal (and simple patch) was first discussed here [1].

Basic idea is the tablesync could/should detect if there is anything
to do *before* it enters the apply main loop. Calling
process_sync_tables() before the apply main loop offers a quick way
out, so the message handling will not be unnecessarily between
workers. This will be a small optimization.

But also, IMO this is a more natural separation of work. E.g tablesync
worker will finish when the table is synced - not go one extra step...

~~

This patch was already successfully used for several versions
(v43-v50) of another 2PC patch [2], but it was eventually removed from
there because, although it has its own independent value, it was not
required for that patch series [3].

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[1] https://www.postgresql.org/message-id/CAHut%2BPtjk-Qgd3R1a1_tr62CmiswcYphuv0pLmVA-%2B2s8r0Bkw%40mail.gmail.com
[2] https://www.postgresql.org/message-id/flat/CAHut%2BPsd5nyg-HG6rGO2_5jzXuSA1Eq5%2BB5J2VJo0Q2QWi-1HQ%40mail.gmail.com#1c268eeee3756b32e267d96b7177ba95
[3] https://www.postgresql.org/message-id/CAA4eK1Jxu-3qxtkfA_dKoquQgGZVcB%2Bk9_-yT5%3D9GDEW84TF%2BA%40mail.gmail.com

Kind Regards,
Peter Smith.
Fujitsu Australia