Allow queries submitted by postgres_fdw to be canceled.

Back-patch of commits f039eaac and
1b812afb, 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