Fail gracefully if a user signs into Firebase twice with the same agent (clientId)

[dborkan]

Fail gracefully if a user signs into Firebase twice with the same agent (clientId). Currently if a the same user signs in twice with the same clientId, messages will get mixed up between the two sign ins and the user may end up in a bad state (e.g. if the first log in signs out, the client will be marked offline even though the 2nd login still thinks it is online).

uProxy can currently work around this by passing a random number as the agent string for each login.

We should fix this by ensuring that a given clientId can only be signed in at one place, either by:

• Rejecting later login attempts for an already online clientId
• Allow later login attempts for an already online clientId, but kicking the first login offline