Three-way merge

[bobvawter]

This is the top-level tracking issue for work related to implementing a three-way merge operation.

Plan:

• [x] #509 (Use this issue for design discussion)
• Receive and persist changefeed before values in mutations
  • [x] #517
  • [x] #520
• Return conflicted rows from current CAS behavior
  • [X] #540
  • [ ] MySQL
  • [ ] Oracle
• Define and invoke merge userscript callback
  • [x] #531
  • [x] #540
  • [x] #547
• DLQ implementations
  • [x] #543
  • [x] #546
  • [X] Document DLQ table schema
• Extended CAS behaviors (if needed by use-cases)
  • [ ] CRDB / Postgres
  • [ ] MySQL
  • [ ] Oracle
• Improve idempotency of resolver loop
  • [X] #566
  • [X] #567
  • [X] #568
  • [x] #572
• Cookbook for active-active / bi-directional replication patterns
  • [ ] #573 (Paused)
  • [x] Create at least an outline of three-way merge doc in the wiki
• Misc PRs
  • [X] #577