Bandwidth limiting

[darkdarkdragon]

What's the problem? (required)
We want to be able to limit bandwidth used by API users.

Proposed implementation
Mist allows to configure webhook that will be triggered when stream exceed some threshold -
we can use that to implement configurable system for limiting bandwidth.

Bandwidth could be constrained per stream and per user (say, we have constrain of 10Mb per stream and 50Mb per user - that means that user will not be able to simultaneously transcode more than 5 streams).
There will be global limit that can be overridden per user.

API changes:
add global object
/globalConfig

{
    bandwithLimit: {
        stream: number, // bytes per second
        user: number // bytes per second
    }
}

Also field bandwithLimit with the same structure could optionally be added to user object.
API endpoint /api/stream/key/:streamKey which is used by mist-api-connector to retrieve
stream's information will also return bandwithLimit inside stream object. First it will
look for bandwithLimit in the user's object, and if not set then will return bandwithLimit
from the globalConfig object. mist-api-connector will use that information to stop streams
that exceed specified threshold.