Use RwLock for state DB

[tomusdrw]

To allow parallel reads.

Had to introduce Mutex in state_db_cache though (since Account has Cell/RefCell inside which makes it !Sync). However the Mutex is guaranteed to be accessed from single thread (withing sync_cache method) and actually Account is never modified (not even read from) in that context, it should be safe to just wrap it with something that provides unsafe Sync, but IMHO better be safe with this.

[arkpar]

Not sure that creating thousands of mutexes is a good idea.

[arkpar]

Parallel reads were actually possible before this change. The mutex was never held for the actual read but only to make a clone.

[tomusdrw]

@arkpar Let me know if this is any better: #7456
if you are not happy with it we can revert the PR.

The mutex was never held for the actual read but only to make a clone.
Indeed that was the case although IMHO it's still worth to relax the requirements by using RwLock.