expose fastify server in websocket handlers

[einnjo]

Exposes fastify server in global and per route websocket handlers via this binding.

Adds tests to ensure this is the case in both kinds of handlers.

Adds a note in README to alert users about fastify request lifecycle expectations and clue them into the exposed server inside han dlers.

The typings for per route handlers seemed to already declare that this was bound to the fastify instance even if in reality that was not the case.

Fixes #66

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

[mcollina]

Would you mind to update https://github.com/fastify/fastify-websocket/blob/master/test/types/index.test-d.ts to test for this type?

[einnjo]

I gave it a go, but I'm completely new to testing types and not familiar at all with tsd. I verified that the tests failed if I removed the declaration from the types file.

[mcollina]

lgtm