[CON-69] Modify existing AUDIT method APIs so that they track auditing history with user attribution

Subtasks:
- [ ] Assign unique uid for each user
- [ ] Add additional param for uid for add and remove methods in all of the Engine stores (i.e. Store, BufferedStore, Limbo, Buffer, etc.)
- [ ] Modify how Write and Revision store things on disk to account for uid
- [ ] Assign UIDs to users in previous versions of Concourse. (Perhaps another upgrade task)
- [ ] Modify stored data to have uid field. (Perhaps another upgrade task)