pgsql: postgres_fdw: Inherit the local transaction's access/deferrable

postgres_fdw: Inherit the local transaction's access/deferrable modes.

Previously, postgres_fdw always 1) opened a remote transaction in READ
WRITE mode even when the local transaction was READ ONLY, causing a READ
ONLY transaction using it that references a foreign table mapped to a
remote view executing a volatile function to write in the remote side,
and 2) opened the remote transaction in NOT DEFERRABLE mode even when
the local transaction was DEFERRABLE, causing a SERIALIZABLE READ ONLY
DEFERRABLE transaction using it to abort due to a serialization failure
in the remote side.

To avoid these, modify postgres_fdw to open a remote transaction in the
same access/deferrable modes as the local transaction. This commit also
modifies it to open a remote subtransaction in the same access mode as
the local subtransaction.

Although these issues exist since the introduction of postgres_fdw,
there have been no reports from the field. So it seems fine to just fix
them in master only.

Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAPmGK16n_hcUUWuOdmeUS%2Bw4Q6dZvTEDHb%3DOP%3D5JBzo-M3QmpQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e5a3c9d9b5ce535151d3a7e3173e8d27d2d8cd58

Modified Files
--------------
contrib/postgres_fdw/connection.c | 99 ++++++++++++++++--
contrib/postgres_fdw/expected/postgres_fdw.out | 134 +++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 78 ++++++++++++++
doc/src/sgml/postgres-fdw.sgml | 15 +++
src/backend/access/transam/xact.c | 28 ++++++
src/include/access/xact.h | 1 +
6 files changed, 347 insertions(+), 8 deletions(-)