Update netmgr, tasks, and applications to use isc_loopmgr

Previously:

* applications were using isc_app as the base unit for running the
  application and signal handling.

* networking was handled in the netmgr layer, which would start a
  number of threads, each with a uv_loop event loop.

* task/event handling was done in the isc_task unit, which used
  netmgr event loops to run the isc_event calls.

In this refactoring:

* the network manager now uses isc_loop instead of maintaining its
  own worker threads and event loops.

* the taskmgr that manages isc_task instances now also uses isc_loopmgr,
  and every isc_task runs on a specific isc_loop bound to the specific
  thread.

* applications have been updated as necessary to use the new API.

* new ISC_LOOP_TEST macros have been added to enable unit tests to
  run isc_loop event loops. unit tests have been updated to use this
  where needed.

doc/design/zone

@@ -186,13 +186,6 @@ Functions:
 	Initiate transfer of the zone from the given server or the
 	primary servers listed in the zone structure.
 
-    dns_zone_maintenance(dns_zone_t *zone);
-
-	Perform any maintenance operations required on the zone
-	* initiate up to date checks
-	* expire zones
-	* initiate ixfr version expire consolidation
-
     dns_zone_locateprimary(dns_zone_t *zone);
 
 	Working from the root zone locate the primary server for the zone.