Explicit multi-message receive

[igorw]

Currently there is a single message event. If there is only one message, the emitted value is that value. If on the other hand, it is a multi-part message, then the emitted value is an array of messages.

This is implicit and possibly confusing. It should be changed so that:

• message is only emitted for single-part messages.
• messages is emitted for all messages and always emits an array.

Alternative naming might be multi-message, multipart-message or just multipart.