pow: add Version for quick-check of metadata state and refactor lock handling

[sorpaas]

This PR adds a Version field into the mining worker, an atomic value that is incremented every time the metadata is changed. It acts as a quick check for mining threads to determine if they need to fetch the metadata again.

The code is also refactored that now it handles all locking internally -- the MiningHandle interface is mostly same as the previous MiningWorker, but it handles locks itself instead of exposing an Mutex.

We keep using an Mutex for build (instead of a RwLock). While this is a one writer, multiple reader case, write priority is much more important than read, because we have to immediately notify miners of a metadata change, otherwise any work done is invalid anyway.

[Wizdave97]

@sorpaas anyway to get this merged quickly, It works and I kinda need it urgently

[andresilva]

Didn't understand where the version will be used though.

[sorpaas]

Didn't understand where the version will be used though.

I added an example here kulupu/kulupu#168. Properly using it can avoid unnecessary data fetch in the mining lifecycle (checking if metadata is updated, computing the pre-mining information, etc) because we know for sure nothing has changed.

[sorpaas]

bot merge

[ghost]

Trying merge.