Move storage.tab to sessionStorage
Move TowTruck.send logic into session.
Remove some superfluous stuff from towtruck.js (stuff that was undocumented or unused options)
Rename to TowTruckConfig_callToStart as a documented way to defer TowTruck initialization
Put all startup state into TowTruck.startup
Make session.js use just that information, and remove some redundant logic
Make clientId be based on identityId and a random per-tab token, so multiple tabs will be multiple clients