Fix DROP TABLESPACE on Windows with ProcSignalBarrier?

Hello,

In tablespace.c, a comment explains that DROP TABLESPACE can fail
bogusly because of Windows file semantics:

* XXX On Windows, an unlinked file persists in the directory listing
* until no process retains an open handle for the file. The DDL
* commands that schedule files for unlink send invalidation messages
* directing other PostgreSQL processes to close the files. DROP
* TABLESPACE should not give up on the tablespace becoming empty
* until all relevant invalidation processing is complete.

While trying to get the AIO patchset working on more operating
systems, this turned out to be a problem. Andres mentioned the new
ProcSignalBarrier stuff as a good way to tackle this, so I tried it
and it seems to work well so far.

The idea in this initial version is to tell every process in the
cluster to close all fds, and then try again. That's a pretty large
hammer, but it isn't reached on Unix, and with slightly more work it
could be made to happen only after 2 failures on Windows. It was
tempting to try to figure out how to use the sinval mechanism to close
precisely the right files instead, but it doesn't look safe to run
sinval at arbitrary CFI() points. It's easier to see that the
pre-existing closeAllVfds() function has an effect that is local to
fd.c and doesn't affect the VFDs or SMgrRelations, so any CFI() should
be an OK time to run that.

While reading the ProcSignalBarrier code, I couldn't resist replacing
its poll/sleep loop with condition variables.

Thoughts?