Remove duplicate logic from Channel framework

• Type: Story
• Status: Backlog
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: None
• Fix Version/s: None
• Component/s: Push Notifications
• Labels: None
• Git Pull Request: https://github.com/shuyangzhou/liferay-portal/pull/835

The Channel framework can be simplified by eliminating some of the convenience methods from the lower level implementation and handling those methods in the ChannelHubManager which is the access point for the channel framework.