Update SPEC with Tributary updates

[yondonfu]

• Update wording in mechanics section: no longer implies that the transcoder pulls segments from the broadcaster since broadcasters push segments to transcoders in the new networking architecture.
• Update job assignment algorithm: no longer implies that the active transcoder assigned to a job is the only entity that would call bondingManager.electActiveTranscoder() to determine the address of the assigned transcoder.
• Incorporate changes in Tributary LIPs (3, 8, 9, 11)
• Added a delegator protocol states diagram and a transcoder protocol states diagram

[j0sh]

LGTM

[dob]

Thanks for all these updates. Looking good.