Bug 1073732: Allow Marionette to have sessions ids that are unique for that session; r=jgriffin


The string is an opaque string(uuid) that uniquely identifies the session. It gives us webdriver compability and we can
use it to set sessions if we are doing restarts

--HG--
extra : rebase_source : e2d59e16a82d431da4a445f9890433353a6ac7f7