Add support for clean shutdown of idle connections

[gwatts]

Spent some time over the weekend address a few issues with manners and adding more tests.

This first branch/PR maintains a backwards compatible interface with the existing implementation and also:

• Stop server from allowing keep-alive requests after shutdown begins
• Remove race conditions from tests
• Remove mutex from listener
• Improve tests and make deterministic
• Maintains interface backwards compatibility

However I did more work in another branch (#22) that goes further at the expense of backwards compatibility.. I'll open a PR for that one separately so you can take a look.

[lionelbarrow]

This is superseded by the work you did on the new API. Thanks for the contribution though, and I apologize for taking so long to get back to you.