pgsql: Allow queries submitted by postgres_fdw to be canceled.

Allow queries submitted by postgres_fdw to be canceled.

This fixes a problem which is not new, but with the advent of direct
foreign table modification in 0bf3ae88af330496517722e391e7c975e6bad219,
it's somewhat more likely to be annoying than previously. So,
arrange for a local query cancelation to propagate to the remote side.

Michael Paquier, reviewed by Etsuro Fujita. Original report by
Thom Brown.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f039eaac7131ef2a4cf63a10cf98486f8bcd09d2

Modified Files
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contrib/postgres_fdw/connection.c | 99 +++++++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c | 123 +++++++++++++++++++++++-------------
contrib/postgres_fdw/postgres_fdw.h | 2 +
3 files changed, 180 insertions(+), 44 deletions(-)