Speculative: Run transaction rollback asynchronously

[josephschorr]

Right now, the transaction used in the CRDB driver to set the query to use the proper time is rolled back synchronously via a defer tx.Rollback(ctx) call in the query code: https://github.com/authzed/spicedb/blob/main/internal/datastore/common/sql.go#L248

During performance testing it was noticed that this rollback can take anywhere from 0.5ms to 3ms, so it might be possible to get a small win if we can run the rollback asynchronously while the queried data is returned to the caller. This may not be possible given the connection pool in use; we'd need to investigate if this would cause any secondary effects.

[ecordell]

looks like it should be fine:

jackc/pgx#866

https://github.com/jackc/pgx/blob/93c6b60429e13e0016665214dca2c6382982cf99/pgxpool/doc.go#L1