Use more consistent names for wait event objects and types

Use more consistent names for wait event objects and types

The event names use the same case-insensitive characters, hence applying
lower() or upper() to the monitoring queries allows the detection of the
same events as before this change.  It is possible to cross-check the
data with the system view pg_wait_events, for instance, with a query
like that showing no differences:
SELECT lower(type), lower(name), description
  FROM pg_wait_events ORDER BY 1, 2;

This will help in the introduction of more simplifications in the format
of wait_event_names.  Some of the enum values in the code had to be
renamed a bit to follow the same convention naming across the board.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZOxVHQwEC/9X/p/z@paquier.xyz