Smart Home: Cache authorization tokens

[bramkragten]

Closes #421

For every requestSync or reportState request, a new JWTClient is created, causing a post request to get a new token:

actions-on-google-nodejs/src/service/smarthome/smarthome.ts

Line 332 in eaa9e20

const jwtClient = new google.auth.JWT(

The token that is returned is valid for an hour, but the client and token are still recreated for every request.

With this PR, the JWTClient is created just once, on first use, this causes that the token is cached and only refreshed when it is no longer valid.

Furthermore, during the request of a new token, all other requests now wait for this token to finish loading instead of starting a new load.

This will drastically improve the performance and lower the number of requests to the Google Authentication API.

(Sorry, prettier doesn't seem agree with your style guidelines)