fix: Don't refresh session on each request

[jack-y]

New option: rolling.
Checks if we force the session identifier cookie to be set on every response.
Default = true.
Same approach as express-session.
See: https://github.com/expressjs/session#rolling

Usage:

fastify.register(fastifySession, {
  ...
  rolling: false, // don't force the session cookie to be set on every response
  ...
});

Hope it helps!

[coveralls]

Coverage decreased (-4.1%) to 95.89% when pulling e11198c on jack-y:options-rolling into e201f78 on SerayaEryn:master.

[mfpopa]

Fixes #126

[mfpopa]

@jack-y Except the refactoring suggestion I have just made, the only other thing missing is changes to the TypeScript types, https://github.com/SerayaEryn/fastify-session/blob/master/types/types.d.ts to add the rolling option as a boolean to the Options interface.

Ah, also missing tests.