pgsql: Support frontend-backend protocol communication using a shm_mq.

Support frontend-backend protocol communication using a shm_mq.

A background worker can use pq_redirect_to_shm_mq() to direct protocol
that would normally be sent to the frontend to a shm_mq so that another
process may read them.

The receiving process may use pq_parse_errornotice() to parse an
ErrorResponse or NoticeResponse from the background worker and, if
it wishes, ThrowErrorData() to propagate the error (with or without
further modification).

Patch by me.  Review by Andres Freund.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2bd9e412f92bc6a68f3e8bcb18e04955cc35001d

Modified Files
--------------
src/backend/libpq/Makefile       |    2 +-
src/backend/libpq/pqcomm.c       |   98 ++++++++------
src/backend/libpq/pqmq.c         |  261 ++++++++++++++++++++++++++++++++++++++
src/backend/utils/adt/numutils.c |    2 +-
src/backend/utils/error/elog.c   |   51 ++++++++
src/include/libpq/libpq.h        |   36 ++++--
src/include/libpq/pqmq.h         |   22 ++++
src/include/utils/builtins.h     |    2 +-
src/include/utils/elog.h         |    1 +
9 files changed, 428 insertions(+), 47 deletions(-)