[Node support] Support async command handlers

[vkorenev]

Please consider adding support for functions that return Promise as command handlers. A lot of Javascript libraries return values asynchronously via promises or callbacks, which can be converted to promises. Using such API in command handlers is not easy at the moment.

[vkorenev]

Failed promises should be handled (related to #333).

[pvlugter]

Command handlers returning promises (with then methods) added in #367

cloudstate/node-support/src/action-support.js

Line 107 in c7651e9

if (typeof userReturn.then === "function") {