[LiskDAO] make objects ephemeral, aka read-only

Summary:
Make any Lisk object ephemeral, aka read-only, so that we can fiddle
around with their state safe in the knowledge that we'll never end up
writing that updated state back to the db.

Test Plan: Added a new test; ran test suite.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1836