We'd like to live in a world where we can retry all the pending
conflicts in a conversation as a batch, which means we don't want to
wipe the identity key before processing each message. Thus, remove that
step from these handlers and encapsulate in a method on the conversation
model.
Ensure that both tryAgain functions return promises, allowing the
application to take appropriate action in the result of success or
failure. This lets us remove all dependency from libtextsecure on
app-level constructs like message objects/ids and the `extenion.trigger`
function.
Corresponding frontend changes to follow in another commit.
When sending an constrct a copy of the PushMessageContent protobuf, add
a SyncMessageContext, and send it to ourselves. Do this for all kinds of
group messages, and individual text/media messages, but not closeSession
messages as the latter are device-specific.
Do not sync messages if we are the primary device, which should only be
supported in development. Normal web clients must be paired with a
android or ios master device, and even in dev, a primary/standalone web
client does not support linking additional devices.
