INT-2934 IntegrationObjectSupport Initialization of ConversionService

- IntegrationObjectSupport: make the "integrationConversionService"
  initialization in the afterPropertiesSet() method
- AbstractMessageRouter: initialize the DefaultConversionService in
  onInit() in case none is set, remove getRequiredConversionService() as
  it has become obsolete.
- AbstractMappingMessageRouter: call super.onInit() in onInit() and
  remove the null check for channelResolver in resolveChannelForName()
- Adjust unit tests to call afterPropertiesSet()

For further details see
- https://jira.springsource.org/browse/INT-2934
- https://jira.springsource.org/browse/INT-2931