Friend request fixes

[Mikunj]

This PR fixes the logic issues with the previously implemented friend request.

I also refactored some things and moved the logic of adding friend requests out into message_receiver and background

The new logic is the following:

isFriend = Whether we are friends
isKeyExchanged = No cipher text received

• If not isFriend then use fallback encryption.
  • This sets the message type to FRIEND_REQUEST.
• For every message sent, if not isKeyExchanged then we attach preKeyBundle to the Envelope.
  • preKeyBundle used to be inside the Envelope content but was moved outside of it.
  • preKeyBundle is always encrypted using the fallback encryption.
• If we get an encrypted message from the other user then set isKeyExchanged to true.

[sachaaaaa]

Great refactoring.
Just a couple of points to address below.