Determine correct behavior when transaction commit or rollback fails

[elefeint]

Simulate failing transaction.

Right now, DatabaseClientReactiveAdapter clears transaction manager at the end of either commit or rollback. If commit/rollback fails, this may not be correct behavior, especially for commit.

Consider whether transaction state should be kept until a successful commit/rollback, and document how end users should recover from such a state.

[elefeint]

#399 is the part of this dealing with a particular retriable state.