pgsql: Clean up shm_mq cleanup.

Clean up shm_mq cleanup.

The logic around shm_mq_detach was a few bricks shy of a load, because
(contrary to the comments for shm_mq_attach) all it did was update the
shared shm_mq state. That left us leaking a bit of process-local
memory, but much worse, the on_dsm_detach callback for shm_mq_detach
was still armed. That means that whenever we ultimately detach from
the DSM segment, we'd run shm_mq_detach again for already-detached,
possibly long-dead queues. This accidentally fails to fail today,
because we only ever re-use a shm_mq's memory for another shm_mq, and
multiple detach attempts on the last such shm_mq are fairly harmless.
But it's gonna bite us someday, so let's clean it up.

To do that, change shm_mq_detach's API so it takes a shm_mq_handle
not the underlying shm_mq. This makes the callers simpler in most
cases anyway. Also fix a few places in parallel.c that were just
pfree'ing the handle structs rather than doing proper cleanup.

Back-patch to v10 because of the risk that the revenant shm_mq_detach
callbacks would cause a live bug sometime. Since this is an API
change, it's too late to do it in 9.6. (We could make a variant
patch that preserves API, but I'm not excited enough to do that.)

Discussion: https://postgr.es/m/8670.1504192177@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6708e447efb5046c95bdcf900b6da97f56f97ae8

Modified Files
--------------
src/backend/access/transam/parallel.c | 6 ++---
src/backend/executor/tqueue.c | 9 ++++++--
src/backend/libpq/pqmq.c | 10 +++-----
src/backend/storage/ipc/shm_mq.c | 43 +++++++++++++++++++++++++++++------
src/include/storage/shm_mq.h | 4 ++--
5 files changed, 51 insertions(+), 21 deletions(-)