Remove ShardManagerMonitor and other gateway cleanup

[kangalio]

The bulk of this PR is removing ShardManagerMonitor. It was just a background task that received ShardManagerMessage's and called the respective ShardManager function. Now, you can just call the respective ShardManager function directly.

The commits about removing ShardManagerMonitor are marked with [SMM]. The other commits do other miscellaneous related clean-up. See commit titles and descriptions for more details.

Largely untested

[mkrasnitski]

Will review this later today. Just quickly noticed that the docs CI job is failing.

[mkrasnitski]

I'd say this looks good, aside from a few tiny nits. Untangling the mess that is the gateway code is definitely a good thing (I say this as someone relatively unfamiliar with gateway internals).

Could you provide a sentence or two describing the breaking changes in this PR, for changelog purposes?

[kangalio]

- Removed `InterMessage`, `ShardClientMessage`, `ShardManagerMessage`, `ShardManagerMonitor`, `ShardManagerError`
- Changed `ShardManager::new` return type from `(_, ShardManagerMonitor)` to `(_, UnboundedReceiver<Result<(), GatewayError>)`
  - Instead of calling `ShardManagerMonitor.run()`, you just wait for the return value to arrive via the `UnboundedReceiver`
- Changed `ShardMessenger::new` parameter type from `Sender<InterMessage>` to `Sender<ShardRunnerMessage>`
  - All shard-relevant actions have been centralized into `ShardRunnerMessage`
- Changed `ShardMessenger::send_to_shard` return type from `Result<_, _>` to `()`
  - Errors are now logged in the function directly
- Changed `ShardQueuer`'s and `ShardRunnerOptions`'s field `manage_tx: Sender<ShardManagerMessage>` field to `manager: Arc<Mutex<ShardManager>>`
  - Instead of invoking actions on `ShardManager` by sending messages through a channel, you now invoke methods on `ShardManager` directly

Since you said "a sentence or two":

`ShardManagerMonitor` has been removed in favor of interacting with `ShardManager` directly.

[kangalio]

Here is some additional context about the removed types:

Removed:

• ShardClientMessage
  • used to send commands to a ShardRunner. But we already had a type for that: ShardRunnerMessage. ShardClientMessage existed solely because some shard runner commands ended up in the ShardManagerMessage enum for mysterious reasons, so ShardClientMessage was an enum over either ShardRunnerMessage or ShardManagerMessage. This PR moves all shard runner commands into ShardRunnerMessage, so you should use that type instead now.
• InterMessage
  • an enum that only ever contained ShardClientMessage. Use that type directly; or rather it's new replacement ShardRunnerMessage.
• ShardManagerMonitor
  • received ShardManagerMessage's and propagated them to the actual shard manager. This redundancy has been removed and you can instead call methods on ShardManager directly now. All places that previously stored a Sender<ShardManagerMessage> now have a Arc<Mutex<ShardManager>>.
• ShardManagerMessage
  • see above
• ShardManagerError
  • duplicated parts of GatewayError and was removed in favor of GatewayError.

Thanks to @tazz4843 for pointing out the lacking explanation