pgsql: Allow discovery of whether a dynamic background worker is runnin

Allow discovery of whether a dynamic background worker is running.

Using the infrastructure provided by this patch, it's possible either
to wait for the startup of a dynamically-registered background worker,
or to poll the status of such a worker without waiting.  In either
case, the current PID of the worker process can also be obtained.
As usual, worker_spi is updated to demonstrate the new functionality.

Patch by me.  Review by Andres Freund.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/090d0f2050647958865cb495dff74af7257d2bb4

Modified Files
--------------
contrib/worker_spi/worker_spi--1.0.sql      |    2 +-
contrib/worker_spi/worker_spi.c             |   26 +++-
doc/src/sgml/bgworker.sgml                  |   54 ++++++-
src/backend/commands/async.c                |    2 -
src/backend/postmaster/bgworker.c           |  206 ++++++++++++++++++++++++++-
src/backend/postmaster/postmaster.c         |   52 +++++++
src/backend/storage/ipc/procsignal.c        |   12 ++
src/include/miscadmin.h                     |    2 +
src/include/postmaster/bgworker.h           |   21 ++-
src/include/postmaster/bgworker_internals.h |    2 +
src/include/postmaster/postmaster.h         |    1 +
src/include/storage/procsignal.h            |    1 +
12 files changed, 368 insertions(+), 13 deletions(-)