Even port and reservations

[arkgil]

This PR adds support for three additional options which may be passed to Client.allocate/2:

• even_port: true which makes the client request even port without reservation
• reserve: true which makes the client request even port with reservation
• reservation_token: <token> which makes the client use previously obtained reservation token for the allocation

[jerboabot]

Ebert has finished reviewing this Pull Request and has found:

• 2 possible new issues (including those that may have been commented here).

You can see more details about this review at https://ebertapp.io/github/esl/jerboa/pulls/111.

[erszcz]

I don't have any change requests or comments. Looks good :+1: