pgsql: Refactor how some aux processes advertise their ProcNumber

Refactor how some aux processes advertise their ProcNumber

This moves the responsibility of setting the ProcGlobal->walwriterProc
and checkpointerProc fields into InitAuxiliaryProcess. Also switch to
the same pattern to advertise the autovacuum launcher's ProcNumber,
replacing the ad hoc av_launcherpid field in shared memory. This can
easily be extended to other aux processes in the future, if other
processes need to find them.

Switch to pg_atomic_uint32 for the fields. Seems easier to reason
about than volatile pointers. There was some precedence for that, as
were already using pg_atomic_uint32 for the procArrayGroupFirst and
clogGroupFirst fields, which also store ProcNumbers.

Todo: We could also replace WalRecv->procno with this, but that's a
little more code churn so I left that for the future.

Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://www.postgresql.org/message-id/818bafaf-1e77-4c78-8037-d7120878d87c@iki.fi

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/842169b6c110f48c9f7a2dc29d0cedcc97c2ff29

Modified Files
--------------
src/backend/access/transam/xlog.c | 3 +--
src/backend/postmaster/autovacuum.c | 19 ++++++++++---------
src/backend/postmaster/checkpointer.c | 14 ++++----------
src/backend/postmaster/walwriter.c | 6 ------
src/backend/storage/lmgr/proc.c | 32 ++++++++++++++++++++++++++++++--
src/include/storage/proc.h | 7 ++++---
6 files changed, 49 insertions(+), 32 deletions(-)