Allow queries submitted by postgres_fdw to be canceled.

Allow queries submitted by postgres_fdw to be canceled.

Back-patch of commits f039eaac7131ef2a4cf63a10cf98486f8bcd09d2 and
1b812afb0eafe125b820cc3b95e7ca03821aa675, which arranged (in 9.6+) to
make remote queries interruptible.  It was known at the time that the
same problem existed in the back-branches, but I did not back-patch
for lack of a user complaint.

Michael Paquier and Etsuro Fujita, adjusted for older branches by me.
Per gripe from Suraj Kharage.  This doesn't directly addresss Suraj's
gripe, but since the patch that will do so builds up on top of this
work, it seems best to back-patch this part first.

Discussion: http://postgr.es/m/CAF1DzPU8Kx+fMXEbFoP289xtm3bz3t+ZfxhmKavr98Bh-C0TqQ@mail.gmail.com