Every subactor in the tree now receives the socket (or whatever the
mailbox type ends up being) during startup and can call the new
`tractor._discovery.get_root()` function to get a portal to the current
root actor in their tree. The main reason for adding this atm is to
support nested child actors gaining access to the root's tty lock for
debugging.
Also, when a channel disconnects from a message loop, might as well kill
all its rpc tasks.
Prepend the actor and task names in each log emission. This makes
debugging much more sane since you can see from which process and
running task the log message originates from!
Resolves#13
Allows for waiting on another actor (by name) to register with the
arbiter. This makes synchronized actor spawning and consecutive task
coordination easier to accomplish from within sub-actors.
Resolves#31