* Added new negotiating friend state to allow delayed sending of offline
messages
* Added ability to flag currently outgoing message as broken in UI
* Reworked OfflineMsgEngine to support multiple receipt types
* Moved resending logic out of the OfflineMsgEngine
* Moved coordination of receipt and DispatchedMessageId into helper
class usable for both ExtensionReceiptNum and ReceiptNum
* Resending logic now has a failure case when the friend's extension
set is lower than the required extensions needed for the message
* When a user is known to be offline we do not allow use of any
extensions
* Added DB support for broken message reasons
* Added DB support to tie an faux_offline_pending message to a required
extension set
* Introduced ToxExt and CoreExt abstraction
* Along with interfaces for mocking and unit testing
* Add "supportedExtensions" concept to Friend
* Dispatch messages to CoreExt instead of Core when friend supports
extended messages
* Only split messages for core when extended messages are unavailable
* Offline message engine/History not altered. Currently only valid for
an existing session after extension negotiation has completed
