Use secure gRPC api's instead of HTTP api's, use discovery, clean up codebase

[romeovs]

This PR:

• Replaces the HTTP API by the gRPC API for the handler
• Uses discovery to discover the correct handler based on app ID, instead of using black magic for finding out regions
• Uses less magic in the data client, making it robuster
• Rewrites the codebase to be type-checked with flow type and linted like the rest of the TTN codebase
• Uses JSDoc in tandem with documentation.js to document the codebase
• Matches the go-app-sdk api better
• Adds full-stack tests that run on Travis

This has some breaking API changes but migrating should be easy.

[romeovs]

Currently, there's something wrong with the gRPC client and it won't connect to discovery.thethings.network:1900 for instance, failing with a:

9356:error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown

I think this is caused by the server only supporting TLSv1.x and the gRPC client wanting to connect using SSLv3 but I can't reproduce it locally. Maybe @htdvisser can have a look?

[johanstokking]

Let's not merge it until that issue gets fixed.

I don't know if we should support SSLv3 though. This issue doesn't happen with other endpoints (e.g. Handler)?

[johanstokking]

@romeovs

• is Hylke's cipher suite update a fix? Travis doesn't seem to swallow it yet
• do we need SSLv3 on Discovery to make this work?
• why is TLS with the Handler working but not with Discovery?

[htdvisser]

• I'm still making some changes in the public discovery server to make it support it
• We don't need SSLv3
• Because the Handler was using ECDSA certificates, while our Let'sEncrypt certificates are RSA

[johanstokking]

Please reopen when tests pass and conflicts are resolved

[romeovs]

The tests are fixed!

[johanstokking]

@romeovs still blocked?

[romeovs]

@johanstokking it should be okay. It's just waiting for a review :)