ProjectRepairer: Sweep stale snapshots before repairing

Delete snapshots older than a day at the start of each repair. A
snapshot directory is deleted in a finally block once its transfer
finishes, which covers every normal path, including failures and
interrupts. It cannot cover an abnormal JVM exit, such as a kill or an
out of memory kill, between creating the hardlinks and deleting them.
The links that survive keep the objects they point at alive, so gc
cannot reclaim the space until someone removes the directory by hand.

Change-Id: Ia49e874ab39cc1eb86d0ca3fa898f2e7b31fa91d