[hf] fixup mr !599 - cleanup SignalReceiver/ContactStore

SignalReceiver:

- make helpers in different branches of `#dispatch` return
  `Unit` instead of `Job?` (we never used the job and it was odd to
  return null from 3/4 of the branches!)
- call `ContactStore#create` with uuid and phone number from
  `#handlePreKeyBundle` (instead of calling overloaded `#create` that
  takes an envelope as arg) to (1) be simpler, (2) make it more
  explicit why we are storing both pieces of data at this
  point in the session lifecycle
- revise the comment for `#handlePreKeyBundle` (it is a "hot spot" and
  likely to be the source of lots of confusion to future us when less
  debugging state is loaded in our heads!)

ContactStore

- revise `ContactStore#hasContact` to be slightly more
  straightforward (if less terse)
- remove overloaded `#create` (which is no longer called)