Add an "Activated Epoch" and an "Acquired Epoch" to Drydock Leases

Summary: Ref T13189. See PHI690. When a lease is first acquired or activated, note the time. This supports better visibility into queue lengths. For now, this is only queryable via DB and visible in the UI, but can be more broadly exposed in the future.

Test Plan: Landed a revision, saw the leases get sensible timestamps for acquisition/activation.

Reviewers: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13189

Differential Revision: https://secure.phabricator.com/D19613