pgsql: Centralize decision-making about where to get a backend's PGPROC

Centralize decision-making about where to get a backend's PGPROC.

This code was originally written as part of parallel query effort, but
it seems to have independent value, because if we make one decision
about where to get a PGPROC when we allocate and then put it back on a
different list at backend-exit time, bad things happen.  This isn't
just a theoretical risk; we fixed an actual problem of this type in
commit e280c630a87e1b8325770c6073097d109d79a00f.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6f2871f12e9fba5deec4296cfe12e85c140261c4

Modified Files
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src/backend/storage/lmgr/proc.c |   55 ++++++++++++++++++---------------------
src/include/storage/proc.h      |    1 +
2 files changed, 27 insertions(+), 29 deletions(-)