pgsql: postgres_fdw: Add option to control whether to keep connections

postgres_fdw: Add option to control whether to keep connections open.

This commit adds a new option keep_connections that controls
whether postgres_fdw keeps the connections to the foreign server open
so that the subsequent queries can re-use them. This option can only be
specified for a foreign server. The default is on. If set to off,
all connections to the foreign server will be discarded
at the end of transaction. Closed connections will be re-established
when they are necessary by future queries using a foreign table.

This option is useful, for example, when users want to prevent
the connections from eating up the foreign servers connections
capacity.

Author: Bharath Rupireddy
Reviewed-by: Alexey Kondratov, Vignesh C, Fujii Masao
Discussion: https://postgr.es/m/CALj2ACVvrp5=AVp2PupEm+nAC8S4buqR3fJMmaCoc7ftT0aD2A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b1be3074ac719ce8073fba35d4c8b52fb4ddd0c3

Modified Files
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contrib/postgres_fdw/connection.c              | 33 ++++++++++++++++++++---
contrib/postgres_fdw/expected/postgres_fdw.out | 23 +++++++++++++++-
contrib/postgres_fdw/option.c                  |  4 ++-
contrib/postgres_fdw/sql/postgres_fdw.sql      | 13 +++++++++
doc/src/sgml/postgres-fdw.sgml                 | 37 +++++++++++++++++++++++---
5 files changed, 101 insertions(+), 9 deletions(-)