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.

bin/tests/system/statistics/tests.sh

@@ -217,7 +217,7 @@ if $FEATURETEST --have-libxml2 && "${CURL}" --http1.1 http://10.53.0.3:${EXTRAPO
     # grep "<h2>Glue cache statistics</h2>" xsltproc.out.${n} >/dev/null || ret=1
     grep "<h3>View _default" xsltproc.out.${n} >/dev/null || ret=1
     grep "<h4>Zone example" xsltproc.out.${n} >/dev/null || ret=1
-    grep "<h2>Task Manager Configuration</h2>" xsltproc.out.${n} >/dev/null || ret=1
+    # grep "<h2>Task Manager Configuration</h2>" xsltproc.out.${n} >/dev/null || ret=1
     grep "<h2>Tasks</h2>" xsltproc.out.${n} >/dev/null || ret=1
     grep "<h2>Memory Usage Summary</h2>" xsltproc.out.${n} >/dev/null || ret=1
     grep "<h2>Memory Contexts</h2>" xsltproc.out.${n} >/dev/null || ret=1