run_check_repl_dbhash_background aborts all transactions after an error even if they've already successfully committed

The dbHash command is run inside of a transaction in run_check_repl_dbhash_background.js on each node in the test's replica set. When committing the transactions, we will iterate over each session (one per node) and call commitTransaction. If any of these calls throws an error, we catch the error and iterate over each session to try to abort each transaction. But if any transaction was already successfully committed, trying to abort it errors with:

Error: Cannot call abortTransaction after calling commitTransaction. :

We should only try to abort transactions that are still active.