Proposal for graceful stop

This pull request enables graceful stop for Tiger Tonic services (or really any service that uses `tigertonic.Server` instead of `http.Server`).  Most of the work is done by `tigertonic.Listener` and `tigertonic.Conn` which account for connections with a `sync.WaitGroup`.  Keepalives are managed by `closingResponseWriter`, which injects a `Connection: close` header into responses written after the listener is closed.

The major open question is whether this should be always-on or optional.