pgsql: Rearrange logrep worker's snapshot handling some more.

Rearrange logrep worker's snapshot handling some more.

It turns out that worker.c's code path for TRUNCATE was also
careless about establishing a snapshot while executing user-defined
code, allowing the checks added by commit 84f5c2908 to fail when
a trigger is fired in that context.

We could just wrap Push/PopActiveSnapshot around the truncate call,
but it seems better to establish a policy of holding a snapshot
throughout execution of a replication step.  To help with that and
possible future requirements, replace the previous ensure_transaction
calls with pairs of begin/end_replication_step calls.

Per report from Mark Dilger.  Back-patch to v11, like the previous
changes.

Discussion: https://postgr.es/m/B4A3AF82-79ED-4F4C-A4E5-CD2622098972@enterprisedb.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/26383da7d47a140c33271bfd9215fe50959c80e3

Modified Files
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src/backend/replication/logical/worker.c | 70 +++++++++++++++++---------------
1 file changed, 38 insertions(+), 32 deletions(-)