Separate requests for certain sets of CIDs in wantlist-client

[mrd0ll4r]

We should have some system to add sets of CIDs to the wantlist-client such that wantlist entries for them get tagged in prometheus.

I'm thinking of maybe a system that watches a given directory and reads lists of CIDs from files.
Something like:

...
+-- watched-dir
     |-- nft.cids.list 
     |-- libgen.cids.list    
     |...
     +-- foo.cids.list

And we tag them in prometheus based on the file name, nft, libgen, etc.
Everything that doesn't match gets some other tag (or none?)

For now we can probably store them (as bytes) in a HashMap.
If things get big (> 1M tracked CIDs, back of the envelope calculation) we could think of something more fancy :)