Add remote context allocation api to service daemon

This allows for more deterministically managing long running sub-daemon
services under `pikerd` using the new context api from `tractor`.
The contexts are allocated in an async exit stack and torn down at root
daemon termination. Spawn brokerds using this method by changing the
persistence entry point to be a `@tractor.context`.

piker/_daemon.py

@@ -18,7 +18,6 @@
 Structured, daemon tree service management.
 
 """
-from functools import partial
 from typing import Optional, Union, Callable, Any
 from contextlib import asynccontextmanager, AsyncExitStack
 from collections import defaultdict