Issue #1076: load commit cache in a background thread

Issue #1076: load commit cache in a background thread

* Make the CommitCache fully thread-safe. It was using a
  ConcurrentHashMap containing lists, but then handed out these lists.
  It also did multiple operations on that map that as a whole should
  be atomic.

* Use isEmpty() instead of size() == 0.

* Run the loading of the commit cache in a background daemon thread