DfsGarbageCollector: Add setter for reflog expiration time.

DfsGarbageCollector: Add setter for reflog expiration time.

JGit reftable writer/compator knows how to prune the history, but the
DfsGarbageCollector doesn't expose the time limit.

Add a method to DfsGarbageCollector to set the reflog time limit.
This value is then passed to the reftable compactor. Callers usually
pass here the value from gc.reflogExpire.

The reflog block length is stored in 24 bits [1], limiting the size to
16MB. I have observed that in repositories with frequent commits,
reflogs hit that size in 6-12 months.

[1] https://git-scm.com/docs/reftable

Bug: jgit-96

Change-Id: I8b32d6d2b2e1d8af8fb7d9f86225d75f1877eb2f