ExecuteInTx does not honor context cancellation

The retry loop in `ExecuteInTx` does not check ctx.Done to abort the retries if the context is cancelled.

Found while working on adding query cancellation in the crdb SQL shell.

cc @andreimatei @rafiss

Start at ExecuteInTx and inspect its retry loop and context handling. Reproduce the cancellation path described while adding query cancellation in the crdb SQL shell, then verify that cancellation stops retries without changing the existing transaction behavior.

Written by the indexing model from the issue text.