Instead of blocking the message queue when we don't have a session, we instead just send out a session request and send the queued messages using fallback encryption.
This means that users will be able to message right away without having to wait.
The only down side is that all messages sent before sessions are established will be using the weaker encryption.
This change also means we have to detach session requests from envelope type (which is a good thing) and thus now a message is a session request if it contains a preKeyBundle.
Mikunj
Vincent