postgres_fdw: Allow cancellation of transaction control commands.

Commit f039eaac, later back-patched
with commit 1b812afb, allowed many of
the queries issued by postgres_fdw to fetch remote data to respond to
cancel interrupts in a timely fashion.  However, it didn't do anything
about the transaction control commands, which remained
noninterruptible.

Improve the situation by changing do_sql_command() to retrieve query
results using pgfdw_get_result(), which uses the asynchronous
interface to libpq so that it can check for interrupts every time
libpq returns control.  Since this might result in a situation
where we can no longer be sure that the remote transaction state
matches the local transaction state, add a facility to force all
levels of the local transaction to abort if we've lost track of
the remote state; without this, an apparently-successful commit of
the local transaction might fail to commit changes made on the
remote side.  Also, add a 60-second timeout for queries issue during
transaction abort; if that expires, give up and mark the state of
the connection as unknown.  Drop all such connections when we exit
the local transaction.  Together, these changes mean that if we're
aborting the local toplevel transaction anyway, we can just drop the
remote connection in lieu of waiting (possibly for a very long time)
for it to complete an abort.

This still leaves quite a bit of room for improvement.  PQcancel()
has no asynchronous interface, so if we get stuck sending the cancel
request we'll still hang.  Also, PQsetnonblocking() is not used, which
means we could block uninterruptibly when sending a query.  There
might be some other optimizations possible as well.  Nonetheless,
this allows us to escape a wait for an unresponsive remote server
quickly in many more cases than previously.

Report by Suraj Kharage.  Patch by me and Rafia Sabih.  Review
and testing by Amit Kapila and Tushar Ahuja.

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