feat: expose BlockKeyCacheSize and enable WriteThrough datastore options

[hsanjuan]

This enables WriteThrough blockstore and blockservice by default, exposing a new option and BlockKeyCacheSize to control two-queue cache and be able to disable it.

[hsanjuan]

We can discuss a bit, because I am not sure when a non-writethrough blockstore/service makes fully sense. Probably on small datastores with very well primed caches that fit all keys and when re-writing the same keys all the time (not sure how common that is to warrant being the current default).

I don't think non-WriteThrough makes sense without bloom-filter. And bloom filter is primed on boot, which triggers reading all the keys (horrible in very large datastores).

• flatfs: I'm guessing the OS provides some caching for folders etc. but Has() calls trigger 2-levels of directory listing on every block-write. Might be better than writing the block IF the block is already there.
• Pebble/Badger: Given they can have 30x read amplification, I am not sure it ever makes sense to touch the read-path at all. They can optimize writes internally as they wish.

[hsanjuan]

Comments have been addressed, changelog has been improved, Boxo has been updated to main and nopfs too as flyby.

[lidel]

Ok to merge once #10631 lands in master + rebase + below refactor lands (I'll do that later today, just dropping comment so we don't forget)