pgsql: Transform proconfig for faster execution.

Transform proconfig for faster execution.

Store function config settings in lists to avoid the need to parse and
allocate for each function execution.

Speedup is modest but significant. Additionally, this change also
seems cleaner and supports some other performance improvements under
discussion.

Discussion: https://postgr.es/m/04c8592dbd694e4114a3ed87139a7a04e4363030.camel@j-davis.com
Reviewed-by: Nathan Bossart

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5765cfe18c595b5d8a7df3a62d253f60a00718ce

Modified Files
--------------
src/backend/utils/fmgr/fmgr.c | 29 +++++++++++++++++++---------
src/backend/utils/misc/guc.c  | 45 ++++++++++++++++++++++++++++++++++++-------
src/include/utils/guc.h       |  2 ++
3 files changed, 60 insertions(+), 16 deletions(-)