pgsql: postgres_fdw: Add support for parallel abort.

postgres_fdw: Add support for parallel abort.

postgres_fdw aborts remote (sub)transactions opened on remote server(s)
in a local (sub)transaction one by one when the local (sub)transaction
aborts. This patch allows it to abort the remote (sub)transactions in
parallel to improve performance. This is enabled by the server option
"parallel_abort". The default is false.

Etsuro Fujita, reviewed by David Zhang.

Discussion: http://postgr.es/m/CAPmGK15FuPVGx3TGHKShsbPKKtF1y58-ZLcKoxfN-nqLj1dZ%3Dg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/983ec23007bd83a649af9bc823f13feb0da27e0e

Modified Files
--------------
contrib/postgres_fdw/connection.c | 403 +++++++++++++++++++++++--
contrib/postgres_fdw/expected/postgres_fdw.out | 53 +++-
contrib/postgres_fdw/option.c | 2 +
contrib/postgres_fdw/sql/postgres_fdw.sql | 27 +-
doc/src/sgml/postgres-fdw.sgml | 47 ++-
5 files changed, 489 insertions(+), 43 deletions(-)