pgsql: Split wait event related code from pgstat.[ch] into wait_event.[

Split wait event related code from pgstat.[ch] into wait_event.[ch].

The wait event related code is independent from the rest of the
pgstat.[ch] code, of nontrivial size and changes on a regular
basis. Put it into its own set of files.

As there doesn't seem to be a good pre-existing directory for code
like this, add src/backend/utils/activity.

Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/20210316195440.twxmlov24rr2nxrg@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a333476b925134f6185037eaff3424c07a9f466f

Modified Files
--------------
src/backend/postmaster/pgstat.c         | 673 -------------------------------
src/backend/utils/Makefile              |  15 +-
src/backend/utils/activity/Makefile     |  19 +
src/backend/utils/activity/wait_event.c | 690 ++++++++++++++++++++++++++++++++
src/include/pgstat.h                    | 274 +------------
src/include/utils/wait_event.h          | 301 ++++++++++++++
6 files changed, 1025 insertions(+), 947 deletions(-)