pgsql: Don't count background workers against a user's connection limit

Don't count background workers against a user's connection limit.

Doing so doesn't seem to be within the purpose of the per user
connection limits, and has particularly unfortunate effects in
conjunction with parallel queries.

Backpatch to 9.6 where parallel queries were introduced.

David Rowley, reviewed by Robert Haas and Albe Laurenz.

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/13752743bf7010f7cbf07bdf2a0087c3960121c5

Modified Files
--------------
doc/src/sgml/ref/create_database.sgml | 3 ++-
doc/src/sgml/ref/create_role.sgml | 5 ++++-
src/backend/access/transam/twophase.c | 1 +
src/backend/storage/ipc/procarray.c | 34 ++++++++++++++++++++++++++++++++++
src/backend/storage/lmgr/proc.c | 2 ++
src/backend/utils/init/postinit.c | 2 +-
src/include/storage/proc.h | 2 ++
src/include/storage/procarray.h | 1 +
8 files changed, 47 insertions(+), 3 deletions(-)