Deduplicating IPC strand messages (#756)

Deduplicating IPC strand messages (#756)

No functionality changes other than minor debugging improvements.

* replaced identical (except for the message kind value) HereIamMessage
  and StrandSearchResponse classes with StrandMessage
* reduced code duplication with a new StrandMessage::NotifyCoordinator()
* split TypedMsgHdr::type() into unchecked rawType() and checked type()
* renamed and documented several Ipc::MessageType enum values

The above code improvements will help with adding more IPC messages.