Provide read and overwrite for Lisk counters
Summary:
Ref T6881. This is part 1 of my 35-step plan to support subscriptions that bill monthly.
Expanding the capabilities of counters will let me use them to create a logical clock on time-based event updates, build a daemon on top of that, and eventually get time-based triggers.
Test Plan: Added and executed unit tests.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: chad, epriestley
Maniphest Tasks: T6881
Differential Revision: https://secure.phabricator.com/D11395