Transitioning to "checking" after start() called

[steveanton]

Currently, the specification for start() step 5 says: "If there are no remote candidates, state remains new." However, there's no language for changing the state in addRemoteCandidate so it does not appear possible to transition out of "new" if start() is called with no remote candidates and some or added later.

Proposal: Always transition the state to "checking" after start() is called, regardless of the number of remote candidates.

Alternatively, if the current behavior is intentional, addRemoteCandidate should have a way to change the state to "checking" if start() has already been called.