Race condition in AutoIncrementTracker can cause two transactions to get assigned the same auto-incrementing ID.

`AUTO INCREMENT` is supposed to be unaffected by transactions: not-yet-committed transactions can still advance the counter, and rolling back a transaction does not reset the counter. This allows two concurrent transactions to both insert into the same table with an `AUTO INCREMENT` column without causing a transaction conflict.

Unfortunately, it appears that in rare cases we can generate the same ID for two concurrent transactions, causing a conflict for whichever transaction commits second.

I was only able to get this to reproduce when the insertion was happening as the result of a trigger.

Steps to reproduce:

```
dolt sql <

Start by running the SQL reproduction against dolt sql-server and compare the two transaction versions of the timestamps table for duplicate primary keys. Trace the AutoIncrementTracker behavior during trigger-driven inserts and concurrent transactions. Done means the reproduction no longer assigns the same auto-incrementing ID to both transactions.

Written by the indexing model from the issue text.