Refactor AbstractHeaderMapper

This commit updates `AbstractHeaderMapper` in a number of ways:

* The 'userDefinedHeaderPrefix' property has been removed as some
  advanced tests revealed that the feature is actually broken right
  now and there is no easy way to fix it. A new richer hook point
  that indicates if the property comes from the target object or the
  standard MessageHeaders has been added.
* A HeaderMatcher interface has been introduced to replace the
  lengthy checks in shouldMapHeader. Several implementations
  of that interface are provided
* An additional pattern has been added that maps any header that is
  *not* a standard header. It uses the standardHeaderPrefix property
  for that purpose
* A number of protected method that were only used to create the
  instance have been removed in favour of a non default constructor.
  Subclasses should provide those *static* values in their own default
  constructor.
* The list of transient headers can now be customized. Transient
  headers are headers that should never be mapped. The standard
  ErrorChannel and ReplyChannel headers have also been removed from
  the default transient headers list as they no longer need to be
  transient
I think we shouldn't remove, but just ramain it as a check to see, that `REPLY_CHANNEL` is mapped as well.
And of courser `assertNotNull` :smile: