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 <[email protected]>
Discussion: https://postgr.es/m/[email protected]

Author: anarazel