Start caching registry requests and integrate with catalog reader

[tomzo]

In LiGet we have following approach for caching, which can be ported to BaGet:

1. Every minute we read events from catalog reader of nuget.org.
2. For any package ID, when an event happens for it, we invalidate cache entries for it.

This allows to cache queries for all versions of packages. BaGet can know all versions of each package, without having to do lookup to nuget.org each time. This allows to serve some queries directly by baget, rather than making extra trip to nuget.org.
In worst case, BaGet does not know about packages which were pushed/unlisted up to 1 minute ago.

This should be done after profiling #9 , we should know if there is actually significant benefit from this.