postgres_fdw: Add support for parallel commit.

postgres_fdw: Add support for parallel commit.

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

Etsuro Fujita, reviewed by Fujii Masao and David Zhang.

Discussion: http://postgr.es/m/CAPmGK17dAZCXvwnfpr1eTfknTGdt%3DhYTV9405Gt5SqPOX8K84w%40mail.gmail.com