fix: wrap dashmap in Arc

[mattsse]

ref #1491

dashmap is clone, which imo is a footgun, because dashmap acts as Rwlock<HashMap> not Arc<Rwlock<HashMap>>

DashMap tries to be very simple to use and to be a direct replacement for RwLock<HashMap<K, V>>. To accomplish this, all methods take &self instead of modifying methods taking &mut self. This allows you to put a DashMap in an Arc and share it between threads while being able to modify it.