pgsql: postgres_fdw: Fix corner cases in transaction mode propagation.

postgres_fdw: Fix corner cases in transaction mode propagation.

This commit addresses the following issues in commit de28140de:

* When the local transaction was DEFERRABLE, begin_remote_xact() started
a remote transaction in the same mode without checking the remote
server version. Since DEFERRABLE is supported only in v9.1 and later,
this would cause a syntax error on older remote servers. Fix this by
propagating the mode only to remote servers that support it.
* pgfdw_xact_callback() committed remote transactions without
synchronizing the read-only mode. If the local transaction had become
READ ONLY since the last remote operation, this would cause deferred
triggers on remote servers to still run in READ WRITE mode. Fix this
by synchronizing the mode at commit time.
* The documentation did not match the actual implementation. Update the
documentation to reflect the implementation.

Reported-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Matheus Alcantara <matheusssilv97(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwE4UMaWGOZq1Z7YUTUSg-CgL4yAzO5YaGwTZe1-6Pjg2g%40mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d6393fc40a59830a88a79bdf08e962122fa9c631

Modified Files
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contrib/postgres_fdw/connection.c | 54 +++++++++++++++++++++++---
contrib/postgres_fdw/expected/postgres_fdw.out | 30 ++++++++++++--
contrib/postgres_fdw/sql/postgres_fdw.sql | 30 ++++++++++++--
doc/src/sgml/postgres-fdw.sgml | 15 +++----
4 files changed, 106 insertions(+), 23 deletions(-)