fix: Deadlocks freezing the whole server after two DHT calls

[SeverinAlexB]

Dashmap xacrimon/dashmap#316 occasionally causes a deadlock on Ubuntu 24. This froze the whole server. Mac would work just fine.

This PR replaces DashMap with a regular Mutex. Slower but works.

[xacrimon]

This isn't a dashmap bug. You're calling entry and storing a RefMut, an exclusive reference equivalent to a write lock in a local variable then calling .await while holding it. This will cause tokio to run another task, say it's running the same function on the same exact thread, causing a standard deadlock, as will any normal lock do.

This is documented in the API, and those guidelines were violated.