pgsql: Add support TCP user timeout in libpq and the backend server

Add support TCP user timeout in libpq and the backend server

Similarly to the set of parameters for keepalive, a connection parameter
for libpq is added as well as a backend GUC, called tcp_user_timeout.

Increasing the TCP user timeout is useful to allow a connection to
survive extended periods without end-to-end connection, and decreasing
it allows application to fail faster.  By default, the parameter is 0,
which makes the connection use the system default, and follows a logic
close to the keepalive parameters in its handling.  When connecting
through a Unix-socket domain, the parameters have no effect.

Author: Ryohei Nagaura
Reviewed-by: Fabien Coelho, Robert Haas, Kyotaro Horiguchi, Kirk
Jamison, Mikalai Keida, Takayuki Tsunakawa, Andrei Yahorau
Discussion: https://postgr.es/m/EDA4195584F5064680D8130B1CA91C45367328@G01JPEXMBYT04

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/249d64999615802752940e017ee5166e726bc7cd

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out |  1 +
contrib/postgres_fdw/sql/postgres_fdw.sql      |  1 +
doc/src/sgml/config.sgml                       | 25 +++++++++
doc/src/sgml/libpq.sgml                        | 14 +++++
src/backend/libpq/pqcomm.c                     | 73 ++++++++++++++++++++++++++
src/backend/utils/misc/guc.c                   | 31 +++++++++++
src/backend/utils/misc/postgresql.conf.sample  |  4 +-
src/include/libpq/libpq-be.h                   |  6 ++-
src/include/utils/guc.h                        |  1 +
src/interfaces/libpq/fe-connect.c              | 43 +++++++++++++++
src/interfaces/libpq/libpq-int.h               |  1 +
11 files changed, 198 insertions(+), 2 deletions(-)