Don't `.aclose` `trio` processes until the very end

Trio will kill subprocesses via `Process.__aexit__()` using a `finally:`
block (which, yes, will get triggered on cancellation) so we avoid that
until true process "tear down" since subactors do many things during
graceful shutdown (such as de-registering from the name discovery
system). Oddly this only seems to be an issue during cancellation of
infinite stream consumption.

Resolves #141

tractor/_spawn.py

@@ -157,7 +157,6 @@ async def cancel_on_completion(
 @asynccontextmanager
 async def spawn_subactor(
     subactor: 'Actor',
-    accept_addr: Tuple[str, int],
     parent_addr: Tuple[str, int],
 ):
 
@@ -167,8 +166,11 @@ async def spawn_subactor(
         # Hardcode this (instead of using ``_child.__name__`` to avoid a
         # double import warning: https://stackoverflow.com/a/45070583
         "tractor._child",
+        # This is merely an identifier for debugging purposes when
+        # viewing the process tree from the OS
         "--uid",
         str(subactor.uid),
+        # Address the child must connect to on startup
         "--parent_addr",
         str(parent_addr)
     ]
@@ -179,9 +181,15 @@ async def spawn_subactor(
             subactor.loglevel
         ]
 
-    async with await trio.open_process(spawn_cmd) as proc:
+    proc = await trio.open_process(spawn_cmd)
     yield proc
 
+    # XXX: do this **after** cancellation/tearfown
+    # to avoid killing the process too early
+    # since trio does this internally on ``__aexit__()``
+    async with proc:
+        log.debug(f"Terminating {proc}")
+
 
 async def new_proc(
     name: str,
@@ -206,7 +214,6 @@ async def new_proc(
         if use_trio_run_in_process or _spawn_method == 'trio':
             async with spawn_subactor(
                 subactor,
-                bind_addr,
                 parent_addr,
             ) as proc:
                 log.info(f"Started {proc}")