Batches serve as a temporary storage for POST'd bsos. Committing a batch
entails moving the bsos from the batch table to their final destination in the
`bsos` table.

The move operation is like an UPSERT: the bsos in the batch will either be
INSERT'd into the `bsos` table or UPDATE'd if they already exist. Only columns
with NON NULL values in the batch table will be UPDATE'd in the `bsos` table:
those with NULL values won't be touched. When INSERTing bsos, columns with NULL
values recieve defaults.

The entire move (batch commit) happens in one single transaction. Spanner
limits the number of writes (or "mutations") within a transaction to 20000
total mutations. A mutation is a write to an individual column. Any writes
requiring modifications to secondary indices also incur additional mutations.
DELETEs are generally cheaper (incurring one mutation per DELETE, not including
secondary indices).

The batch commit mutations are as follows:

- Ensure a parent record exists in `user_collections` (due to bsos INTERLEAVE
  IN PARENT `user_collections`): 4 mutations (quota: False) or 6 mutations
  (quota: True)
  - INSERT or UPDATE:
    - 3 key columns
    - quota: False
      - 1 non key column
    - quota: True
      - 3 non key columns

- Possibly direct inserts via post_bsos (but these only reduce total mutations
  by avoiding writing to the batch table, so not included here)

- Write 1664 (MAX_TOTAL_RECORDS) to `bsos`: max 19968 (1664 * 12) mutations
  - INSERT takes 10 mutations:
    - 4 key columns
    - 4 non key columns
    - INSERT into 2 secondary indices: 2 mutations
  - UPDATE takes 12 mutations:
    - 4 key columns
    - 4 non key columns
    - UPDATE of 2 secondary indices: Each requires deleting + inserting a row:
      2 mutations each. 4 total

- Delete the batch
  - DELETE incurs 1 mutation

- Update `user_collections` quota counts (only when quota: True): 6 mutations
  - UPDATE:
    - 3 key columns
    - quota: True
      - 3 non key columns

Totals:
- quota: False
  4 + 19968 + 1 = 19973

- quota: True
  6 + 19968 + 1 + 6 = 19981
