Cache API calls to fetch watching repos

[pocke]

This change improves the performance of SystemStreamWatchingClient.buildSearchQueries.

Problem

SystemStreamWatchingClient.buildSearchQueries is a bottleneck in my environment because I have 1000+ watching repositories so it calls the API 10+ times.
buildSearchQueries is called for each iteration, which means it is called many times.

Solution

Cache the API calls. By this change, buildSearchQueries returns from the cache, and the cache expires in 10 minutes.

It has also a disadvantage, which it introduces a 10mins delay to update the watching repositories.
But I think it is not a problem. Probably, the users don't expect to synchronize the watching repositories immediately. So the delay is acceptable.

This change has a large impact on me. I'd be very happy if this change is accepted.