refactor: Change concurrency approach (#100)

## Description

Before, the concurrency approach for slots / handles for individual
hashes was based on a top level task pool and "slots" that were managed
in a map in the main actor. There were some tricky race conditions for
the case where a handle would be "revived" when it was already executing
its Drop fn - which does crucial things like writing the bitfield file.
Also, there was actual parallelism per hash, which is probably not
beneficial at all.

Now basically each hash runs effectively single threaded, meaning that
we can later go from actual mutexes to more lightweight synchronisation
primitives like https://crates.io/crates/atomic_refcell . Unfortunately
everything must still be Send due to the fact that we run this whole
thing on a multi-threaded executor 🤷 , thank you tokio. Otherwise we
could just use a
[RefCell](https://doc.rust-lang.org/std/cell/struct.RefCell.html).

Now the concurrency is based on a task pool that will always contain at
most a single task per hash. Multiple tasks that operate on the same
hash are being handled concurrently, but not in parallel, using a
`FuturesUnordered`. The drop case is handled in a cleaner way - when an
actor becomes idle, it "gives back" its state to the owner - the manager
actor.

If a task is being spawned while drop runs, these tasks go into the
inbox and the actor gets revived immediately afterwards. The manager
also has a pool of inactive actors to prevent reallocations.

All this is abstracted away by the entity_manager.

The entire entity_manager module could at some point become a separate
crate, but for now I have inlined it so I don't need to do another
crate.

## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

## Notes & open questions

<!-- Any notes, remarks or open questions you have to make about the PR.
-->

## Change checklist

- [ ] Self-review.
- [ ] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [ ] Tests if relevant.
- [ ] All breaking changes documented.

Author: rklaehn

Cargo.lock

@@ -58,6 +58,7 @@ testresult = "0.4.1"
 tracing-subscriber = { version = "0.3.19", features = ["fmt"] }
 tracing-test = "0.2.5"
 walkdir = "2.5.0"
+atomic_refcell = "0.1.13"
 iroh = { version = "0.91", features = ["discovery-local-network"]}
 
 [features]

Cargo.toml

@@ -4,6 +4,7 @@ use iroh_metrics::{Counter, MetricsGroup};
 
 /// Enum of metrics for the module
 #[allow(missing_docs)]
+#[allow(dead_code)]
 #[derive(Debug, Default, MetricsGroup)]
 #[metrics(name = "iroh-blobs")]
 pub struct Metrics {