pgsql: Remove the restriction that the relmap must be 512 bytes.

Remove the restriction that the relmap must be 512 bytes.

Instead of relying on the ability to atomically overwrite the
entire relmap file in one shot, write a new one and durably
rename it into place. Removing the struct padding and the
calculation showing why the map is exactly 512 bytes, and change
the maximum number of entries to a nearby round number.

Patch by me, reviewed by Andres Freund and Dilip Kumar.

Discussion: http://postgr.es/m/CA+TgmoZq5%3DLWDK7kHaUbmWXxcaTuw_QwafgG9dr-BaPym_U8WQ%40mail.gmail.com
Discussion: http://postgr.es/m/CAFiTN-ttOXLX75k_WzRo9ar=VvxFhrHi+rJxns997F+yvkm==A@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d8cd0c6c95c0120168df93aae095df4e0682a08a

Modified Files
--------------
doc/src/sgml/monitoring.sgml            |  4 +-
src/backend/utils/activity/wait_event.c |  4 +-
src/backend/utils/cache/relmapper.c     | 94 +++++++++++++++++++--------------
src/include/utils/wait_event.h          |  2 +-
4 files changed, 58 insertions(+), 46 deletions(-)