WIP: For your consideration: Transaction fix

[johanix]

Not ready for commit, but I'd appreciate feedback on this change.

• It removes a ton of error returns from all over the music/*ops.go code
• It is based on the observation that transactions can only be initiated by either a request via apiserver or by a mdb.PushZone() from fsmengine. So let's create the transactions there only and remove all that cruft from the rest of the code.
• Happiness is a negative diff :-)

Note that this is only briefly tested at this stage. Although the fix is simple in theory, it is quite large, so it really requires more testing.

Johan