After a failed commit, the session transaction state becomes idle
but the cluster transaction is not cleaned up. A subsequent setPartitionKey
fails because there is still a cluster transaction.

SessionImpl:
  change commit to fail the transaction on errors
  change executeCommit to call flush with the commit flag
  change flush to allow the caller to executeCommit instead of executeNoCommit
    followed by executeCommit
    serendipity: this has a slight performance benefit

TransactionErrorSetPartitionKeyTest
  Add four tests for setPartitionKey after a failure to insert a duplicate record
    commit: this is the original bug report
    flush/commit: flush the failed change and then commit the transaction
    flush/rollback: flush the failed change and then roll back the transaction
    rollback: roll back the failed change