Introduce lock striping

[AlexandraRoatis]

Suggested by @qoire in #300 as follows:

Perhaps we could introduce lock striping either at this level or in a layer below this. The rationale being:

• Assumption is that some block queries are random, therefore may not be hitting blocks that may have high contention (for example the latest n blocks) where n refers to the sync limit that we can rebranch to. Therefore we do not necessarily have to acquire a write lock to the whole object. Merely a write lock to the last 128 blocks. Processes that want to access older historical blocks are still free to do so.

• We can tune this to our API requirements, for example, the API has a method that scans the last n blocks within a block range looking for events.

https://stackoverflow.com/questions/16151606/need-simple-explanation-how-lock-striping-works-with-concurrenthashmap

[qoire]

Thoughts on revisiting this? Still think this is a good idea.

[AlexandraRoatis]

very low priority atm, but would be useful as the database grows and we have a better idea of API requirements

[qoire]

@kzeine candidate for bounty