switch to using isc_loopmgr_pause() instead of task exclusive

switch to using isc_loopmgr_pause() instead of task exclusive

change functions using isc_taskmgr_beginexclusive() to use
isc_loopmgr_pause() instead.

also, removed an unnecessary use of exclusive mode in
named_server_tcptimeouts().

most functions that were implemented as task events because they needed
to be running in a task to use exclusive mode have now been changed
into loop callbacks instead. (the exception is catz, which is being
changed in a separate commit because it's a particularly complex change.)