chg: dev: Use single logging context for everything

Instead of juggling different logging context, use one single logging
context that gets initialized in the libisc constructor and destroyed in
the libisc destructor.

Closes #4848

Merge branch '4848-refactor-isc_log-api-to-use-single-global-logging-context' into 'main'

Closes #4848

See merge request isc-projects/bind9!9301