pgsql: Remove set_latch_on_sigusr1 flag.

Remove set_latch_on_sigusr1 flag.

This flag has proven to be a recipe for bugs, and it doesn't seem like
it can really buy anything in terms of performance. So let's just
*always* set the process latch when we receive SIGUSR1 instead of
trying to do it only when needed.

Per my recent proposal on pgsql-hackers.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/db0f6cad4884bd4c835156d3a720d9a79dbd63a9

Modified Files
--------------
src/backend/postmaster/bgworker.c | 82 +++++++++++-----------------------
src/backend/storage/ipc/procsignal.c | 11 +----
src/backend/storage/ipc/shm_mq.c | 76 +++++++++++++------------------
src/backend/tcop/postgres.c | 4 +-
src/include/storage/procsignal.h | 1 -
src/test/modules/test_shm_mq/setup.c | 65 +++++++++++----------------
6 files changed, 86 insertions(+), 153 deletions(-)