In https://github.com/sourcegraph/sourcegraph/pull/18933 we accidently created a new DefaultRepos store on every
request. This lead to the cache being recreated on every request => no
caching. In practice this increased all global queries on
sourcegraph.com by about 3s. This commit adds back the global caching.
Additionally I took the liberty clean up DefaultRepos. I'm not sure why,
but it is part of the database package. However, it doesn't directly
interact with the database, it interacts with the repos store. So I
extracted the caching logic out into a search specific repos
package. The effect is we no longer have a DefaultRepos store. Instead
we just have a Repos store and a structure which can cache the response
of Repos.ListDefault.
See below for regression on one of our continuous queries on sourcegraph.com:
