To improve message reliability, we're adding an urgency flag to outgoing
messages. For example, outgoing calls are urgent, but delivery receipts
are not. [Android][] and [Desktop][] have already done this work.
At a high level, I added an `isUrgent` property to `TSOutgoingMessage`s.
It defaults to `true`. Some subclasses, like
`OWSReceiptsForSenderMessage`, override it to return `false`.
Builds off of a few other commits (not necessary to understand this
commit, but might be useful for posterity):
- e858a0d916
- 8c6d2ebe8c
- 402b117221
- 2ba0cd764d
- 266a4663e9
- 8e5009bbf7
[Android]: dc04c8ed98
[Desktop]: 06190b1434
da322d3bf9