DfsGarbageCollector will now enforce a maximum time to live (TTL) for
UNREACHABLE_GARBAGE packs. The default TTL is 1 day, which should be
enough time to avoid races with other processes that are inserting
data into the repository.
Change-Id: Id719e6e2a03cfc9a0c0aef8ed71d261dda14bd0c
Signed-off-by: Mike Williams <miwilliams@google.com>