Implement auto join policy handling

[hensm]

Sessions currently do not persist across page loads or multiple tabs. Auto join policy is ignored/treated as page-scoped.

• Should properly keep track of session lifetimes in cast manager and associate with initial origin & content context.
• When creating cast instances, should check against existing sessions with the same app ID at the same origin / tab ID and handle auto join behaviour appropriately.
• Should re-work how sendMessage is implemented so bridge instance can be re-used without scoping messages.

[hensm]

3eba371