pgsql: Replace the sorted array of GUC variables with a hash table.

Replace the sorted array of GUC variables with a hash table.

This gets rid of bsearch() in favor of hashed lookup.  The main
advantage is that it becomes far cheaper to add new GUCs, since
we needn't re-sort the pointer array.  Adding N new GUCs had
been O(N^2 log N), but now it's closer to O(N).  We need to
sort only in SHOW ALL and equivalent functions, which are
hopefully not performance-critical to anybody.

Also, merge GetNumConfigOptions() into get_guc_variables(),
because in a world where the set of GUCs isn't fairly static
you really want to consider those two results as tied together
not independent.

Discussion: https://postgr.es/m/2982579.1662416866@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3057465acfbea2f3dd7a914a1478064022c6eecd

Modified Files
--------------
src/backend/utils/misc/guc.c         | 385 ++++++++++++++++++++++-------------
src/backend/utils/misc/guc_funcs.c   |  39 ++--
src/backend/utils/misc/help_config.c |   5 +-
src/include/utils/guc.h              |   1 -
src/include/utils/guc_tables.h       |   2 +-
5 files changed, 272 insertions(+), 160 deletions(-)