Fixed item should only apply once

Today, our FIXED items are being invoiced whenever a new Plan containing such FIXED item is active. We had some requests to make this behavior configurable and possibly disable any FIXED item when changing Plan.

See #43

Changing the code to exclude the generation of fixed price when changing plans should be easy to implement, this is just a matter of changing the code here to also exclude CHANGE.

However:

• Is this a change of behavior or a configuration -- and if so how?
• Is this only for CHANGE or does it also include TRANSFER ?

See also https://github.com/killbill/killbill/issues/1395

Pausing a subscription with a fixed item also causes the fixed item to be invoiced again once the subscription is resumed. Steps to reproduce:

1. Create a subscription to a plan that has both a fixed and recurring price (See attached catalog as an example) -The invoice generated contains both a FIXED and a RECURRING invoice item.
2. Pause the subscription - A credit is generated only corresponding to the RECURRING item
3. Move the clock by a few days and resume the subscription - The invoice generated again contains a FIXED invoice item.

fixed-and-recurring-price-evergreen-phase.zip