Allow to configure/disable the datastore's ARC cache.

[MichaelMure]

Require ipfs/go-ipfs-config#41

This PR allow to change the size or disable the ARC cache in the datastore.

[MichaelMure]

Test failure seems unrelated.

[MichaelMure]

I'm missing a line to actually pass the config value, will add soon.

[MichaelMure]

Fixed, but still require ipfs/go-ipfs-config#41 to build.

[MichaelMure]

@Stebalien what about this one ?

[Stebalien]

Unfortunately, I'm not sure how to add this in a backwards compatible manner (yet). If we add this as-is, the ARC cache will be disabled on all existing nodes as ARCCacheSize will default to 0.

We can ship a migration but I'd hate to do that. I believe the best way to do this is #8776.

[MichaelMure]

Make sense to me.

[Stebalien]

Revisiting this, we can make progress on this without rewriting the config system. We now have types like config.Flag where we can encode both some type (e.g., a size as a string) or the "default" (null).

Basically, we'd need a new special "size" type with a default option.