Support MySQL

[sorentwo]

This adds support for MySQL databases via a new Dolphin engine and the myxql driver. This is currently a WIP because of the numerous incompatibilities with how Postgres (and even SQLite3) operate.

Progress

• [x] Migration
• [x] Peer Support
• [x] Engine
  • [x] Insert
  • [x] Insert All
  • [x] Staging
  • [x] Pruning
  • [x] Fetching
  • [x] Uniqueness
  • [x] Optimizations
• [x] CI Testing
• [x] Documentation