Return unsubscribe function from each event handler

[pkhien95]

Each event handler should return an unsubscribe function so that client can handle it correctly, especially in React.
For example:

const unsubscribe = messaging.onMessage(function () {
        fetchInboxCount()
})

// Unsubscribe when that screen is not rendered anymore:
unsubscribe()

[tahpot]

This is now implemented:

const listener = function() {
 // do something
}
const handler = messaging.onMessage(listener)
handler.removeListener('newMessage', listener)