refactor: first-pass at runtime interface, signals, shutdown, and more (

#180)

This work precedes subsequent work on running workflows from the
Homestar runner/server/runtime but sets everything into gear.

Includes:

* overall refactoring of events and event handling, streamlining how to
work with senders, etc
* introduce a runner which can
- interact with task/batch specific-abort handles introduced in joinsets
within a worker
- interact with commands, which will happen through the CLI interface,
i.e. star, run-workflow, akin to something like temporal-server
- start-up event handler, websocket server, takes in db and can mock via
implementation
* lots of cleanup(s)

.envrc

@@ -1,5 +1,5 @@
 use_flake
 
-export RUST_LOG=homestar_runtime=debug,atuin_client=warn,libp2p=info,libp2p_gossipsub::behaviour=debug
+export RUST_LOG=homestar_runtime=debug,libp2p=info,libp2p_gossipsub::behaviour=debug
 export RUST_BACKTRACE=full
 export RUSTFLAGS="--cfg tokio_unstable"