Add missing bits

Given very limited amount of time I've approached this with the following hypothesis:

• I moved what was apparent that was added in 30-integrate-firestore and missing in firestore-rebuild
• Skipped the unclear (to me) bits: Here's a list of what I definitively skipped:
  • ignored indexers refactor
  • comparisons.py
  • gcloudc/db/backends/common/context.py
  • dnf changes !51 (diffs)
  • meta queries !51 (diffs)
• Fixed the code to make it work with the refactor

Additional fixes/updates:

• I fixed some issues with unique constraints, but not sure if in the "right" way. Please check 8ce01e444ad758d942576407400beba6f2dcb97b and 658add9a547a17dda11c44d06641a7e67a47ccdd

• I tweaked a test for Firestore that was sporadically failing. Please check 934c708dae950cdc416342936dc607b5efefecce

• Fix issues with anchestor keys and modify tests to catch the problem