Replace the housekeeping thread with a loop owned task

Housekeeper dates from when this work was a blocking select loop. Under
asyncio the event loop is already that place, and a thread around it takes a
captured loop reference, a scheduling step before the thread starts, and a
daemon flag, only to sit waiting on a coroutine that never returns with no way
to stop it. A task runs on the right loop by construction and cancels. The
loop is looked up before the coroutine is built, so calling this without one
raises rather than stranding it.

Nothing in this tree used the class. Out of tree callers need
start_housekeeping() instead, and gain the ability to stop it.

confluent_server/aiohmi/ipmi/command.py

@@ -19,7 +19,6 @@ from itertools import chain
 import os
 import socket
 import struct
-import threading
 
 import asyncio
 import aiohmi.constants as const
@@ -107,19 +106,20 @@ def _cidr_to_mask(prefix):
     return struct.pack('>I', 2 ** prefix - 1 << (32 - prefix))
 
 
-class Housekeeper(threading.Thread):
-    """A Maintenance thread for housekeeping
+def start_housekeeping():
+    """Run aiohmi's recurring work on the current event loop
 
-    Long lived use of aiohmi may warrant some recurring asynchronous behavior.
-    This stock thread provides a simple minimal context for these housekeeping
-    tasks to run in.  To use, do 'aiohmi.ipmi.command.Maintenance().start()'
-    and from that point forward, aiohmi should execute any needed ongoing
-    tasks automatically as needed.  This is an alternative to calling
-    wait_for_rsp or eventloop in a thread of the callers design.
+    Long lived use of aiohmi may warrant some recurring asynchronous
+    behaviour.  Call this once, from the loop the sessions belong to, and
+    aiohmi will service them as needed.  Cancel the returned task to stop.
+    This is an alternative to calling wait_for_rsp or eventloop from the
+    caller's own code.
     """
 
-    def run(self):
-        Command.eventloop()
+    # The loop is fetched first so that calling this without one raises
+    # before the coroutine exists, rather than leaving it unawaited.
+    loop = asyncio.get_running_loop()
+    return loop.create_task(Command.eventloop())
 
 
 class Command(object):