SHDP-334 Refactor generic partition keys

- Generic which used a Map now uses a Spring
  Message which allows to evaluate payload
  together with headers.
- Spel context is now cached so that it wont
  be re-created with every data write. At a same
  time spel variable registration were removed to
  be able to use same context. 'headers', 'payload'
  and header fields can still be accessed with
  shortcuts.