Uplift from mailgun/manners fork

[horkhe]

This PR tries to bring all features implemented in the mailgun/manners fork to the upstream so that projects like vulcand can stop depending on the fork and start using the upstream.

The changes are:

• Added support for TLS listener hijack and update;
• Added capability to specify a state handler that if provided is called on connection state transitions and passed both old and new connection state;
• Added GetFile function that returns a listener socket file handler that can be used to handover to a child process during hot server code reload;
• Added a couple of New... functions for convenience;

The changes have been originally implemented by @klizhentas, I have just rebased them to the upstream master.

[horkhe]

@lionelbarrow please take a look

[horkhe]

It has been more then a month since the PR was proposed. Is anybody maintaining this repo?

[horkhe]

Third time's the charm?