Investigate database deadlock inserting ratelimits

[chronark]

Grafana alerted on a database error when inserting into the ratelimits table.

Error snippet:

database failed to insert ratelimits: Error 1213 (40001): target: unkey.-.primary: vttablet: Deadlock found when trying to get lock; try restarting transaction (errno 1213) (sqlstate 40001)

This happens on an INSERT ... ON DUPLICATE KEY UPDATE into ratelimits with multiple value tuples.

[linear]

ENG-2294