Proposal: Use electrolysis repo as fennec 2 "project branch"

I'm proposing to make e10s (hg.m.o/projects/electrolysis) be to m-c for 
fennec2 work as tracemonkey is to m-c for Spidermonkey work.  The m-c 
and e10s repos would be regularly merged both ways, ~weekly.  This would 
entail new restrictions on e10s

  - all patches pushed to e10s must have required r/sr; that is, be 
ready to land on m-c at any time
  - pushes restricted to only FF4 beta2/fennec2 blocking and approved 
patches
  - m-c checkin rules apply: don't push over unknown orange

In exchange, we get

  - lower-overhead pushes: less contention, eat less unrelated bustage
  - baking time off the m-c critical path
  - tryserver'ing not required before pushing, though recommended

Unless anyone objects, I'll update the e10s tinderbox message and go 
forward with this.

Cheers,
Chris