Handle multiple pending spawns to/from the same app

[wejendorp]

Describe the bug
Calling app.spawn() multiple times creates multiple requests and iframes via the hub.

To Reproduce
Code or link to code to reproduce the behavior:

const app = ts.io();
app.spawn('Tradeshift.MyApp');
app.spawn('Tradeshift.MyApp');
app.spawn('Tradeshift.MyApp');

3 separate iframes w/ MyApp will be created. (UX issue)

Callback behavior is unclear. (integrity issue)

Expected behavior
A clear and concise description of what you expected to happen.

Subsequent spawns should error or (if possible) invalidate the existing request.
Similarly to xhr cancellation.

Additional context
N/A