Cache more things in the DB so we don't hit the disk

[f-f]

Now that we have a database, we should not be reading and writing caches on disk, but just add a database table to have them into

[f-f]

We should also cache the results of getManifestFromIndex since those are immutable. (getMetadata is not, since things can get unpublished)

[JordanMartinez]

@finnhodgkin If you're interested, this may be another one worth looking into.

[f-f]

The part where we cache the result of getManifestFromIndex in the database is still very much needed - basically we need a manifest table that whose primary key is a composite of Package name + Version. We'd leave the call sites untouched, and try to look things up in the DB when we need them, going to the disk only when we don't find them in cache

Since we are going to have lockfiles for all builds (see #917), I don't think the part where we cache package sets in the database makes sense anymore

[finnhodgkin]

@finnhodgkin If you're interested, this may be another one worth looking into.

I'm not sure how much free time I'll have in the next week or two but happy to take a look if I find some.

[f-f]

Something else to pile on this "bring things in the database": right now we write a file and check its modification time to decide if we should be fetching the registry repo. This is slow, and not portable between OSs, so we should just have a table for it.