transcoding: separate pause&stop on transcoder<->scheduler interface

Separate stop from pause, and use stop when a job is being cancelled.
This tells the transcoder to discard the job and all states for it.
Also send the original request to the transcoder start.

bug: 154734285
bug: 145233472
test: unit tests
Change-Id: I112c08b909f3432dfd4b4803c0786a1ea01deea6