Allow testing delayed job queuing synchronously
ProcessToken for tracking async payment operations

* ProcessToken is required bo be unique per process job via indexes
making it a lock preventing multiple execution of 'same' process.
Clean payments implementation

* Pull lookup helpers to a support module
* Make it clear in every API method what it actually does
* Group API operation helpers and error handlers and visually
separate them in source.
Start async support for request api operation
Async running of paymnets.request
Async running of payments.create
Async running of payments full_capture and void
Make the command processing reusable across APIs
Test & fix billing_agreements/charge_commission
Async support for ba/charge_commission
Ensure only one job is scheduled per op
Merge pull request #744 from sharetribe/async-payments

Async execution of paypal payment actions
Change commissions to be digital goods
Dont charge too small commissions
Move all commission calculation logic to transaction