chg: dev: Change the loopmgr to be singleton

All the applications built on top of the loop manager were required to
create a single instance of the loop manager.  Refactor the loop
manager not to expose this instance to the callers, and keep the loop
manager object internal to the `isc_loop` compilation unit.

This significantly simplifies a number of data structures and calls to
the `isc_loop` API.

Merge branch 'ondrej/refactor-isc_loopmgr-to-be-singleton' into 'main'

See merge request isc-projects/bind9!10733