Make result queue poll for shutdown, and tidy up at shutdown

This poll happens at the configured htex poll period, which defaults to
10ms.

Under heavy result load, this shoudn't result in much additional load:
the poll loop will already be looping a lot to process the results.

Under lower result load, there is a slight observable increase in CPU
usage: a 30second sleep task shows this before this PR:

before:
real    0m37.451s
user    0m2.160s
sys     0m0.376s

run 2, user 2.160s
run 3, user 2.116s

and this after this PR:

real    0m37.473s
user    0m2.400s
sys     0m0.557s

Run 2, 2.457s
Run 3, 2.452s

At shutdown, the ZMQ socket for incoming results is closed.

This reduces both the number of threads and number of file descriptors
left behind by the `--config local` tests. For example:

$ pytest parsl/tests/test_monitoring/ --config local

Before this PR, at end of test: 32 threads, 451 fds open.

After this PR, at end of test: 1 thread, 48 fds open.

This is part of PR #3397 shutdown tidyup.

Author: benclifford