Make catfile cache size configurable

Part of #1677 (closed)

Unix resource limits may prevent us from keeping around a lot of subprocesses, particularly on developer MacBooks where the default is 256 open files per process. Making the cache size configurable allows us to accommodate such systems.

As a follow-up step we will have to change the Gitaly configuration in GDK and the gitlab-ce test suite; that is why this MR leaves #1677 (closed) open.

Working on this revealed some problems in how we initialize application configuration during testing so we also change how that works. More things now happen automatically during config.Validate(), and this function is now run both during tests and normal operation. Previously it would not run during tests.