Replace tokio Mutexes with sync Mutexes

[lipanski]

Closes #166

This MR replaces the tokio Mutexes/RwLocks with the sync Mutex/RwLock which is preferred as long as the sync Mutex is not held across await calls.

Since the server state is now wrapped in a sync Mutex, there's no need to call block_on inside the Drop implementations any more.

It also replaces the tokio oneshot channel that shares the server port with a sync mpsc channel. This was the cause for the server pool blocking on futures::future::join_all.