rdbms: make cancelAtomic() handle callbacks and work with DBO_TRX

rdbms: make cancelAtomic() handle callbacks and work with DBO_TRX

Make transaction callbacks aware of cancelled sections. If the
statements of a section are reverted via cancelAtomic(), then the
dependant callbacks are now cancelled as well. Any callbacks for
onTransactionResolution(), which does not depend on COMMIT, will
see the triggering event as a ROLLBACK, since the unit of work it
was part of was rolled back.

Also fix the handling of topmost atomic sections with DBO_TRX.
These still need their own savepoint to make cancelAtomic() work.

Follow-up to 52aeaa7a5.

Change-Id: If4d455c98155283797678cfb9df31d5317dd91a2