Fix lead transaction cancellation

We have a thinko around recording frontend transaction, which leads
to us always cancelling the entire chain. This is not correct, as
evidenced by existing tests.

Correct the book keeping and refactor cancelTransaction() to allow
returning correct result of operation and handle the case where backend
refuses to cancel.

JIRA: MDSAL-756
Change-Id: I553e4de2d09acedf67af63bc292fae0bb5dfed78
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>