Use dashmap for torrent in memory storage instead of the std hashmap

[mickvandijke]

DashMap should be significantly faster in concurrent reads and writes compared to the std HashMap.

https://github.com/xacrimon/dashmap
https://github.com/xacrimon/conc-map-bench

[josecelano]

Relates to: #566

Hi @WarmBeer if you are going to reimplement this, maybe you could also consider trying to avoid this problem.