Store Perforce Branch Details

At present we do not store the details of perforce branches, only their modification times. This means that on startup we need to do "branch -o" on every branch, even those which we know haven't changed, or which may not even be in the repo we are scanning.

To improve the speed of the first scan after startup, we should persist the branch details, so that only the branch update times and any changed branches need to be retrieved from the repository.

Note well that the first startup after upgrading to a version of FishEye/Crucible containing this bug fix will not be faster than previously, as the branches will still need to be loaded. Subsequent startups will be faster.