get rid of Box inside Heartbeat as well

[swizard0]

It turned out to be small, but quite confusing refactoring.

1. At first I changed Box-ed future to a generic type parameter P inside Heartbeat.
2. Then moved new constructor out of Heartbeat impl in order to allow choosing P on the call site.
3. Then moved heartbeat_pulse invocation from the constructor directly to the connect fn.
4. And finally managed to craft connect return type like this:

impl Future<Item = (Self, Heartbeat<impl Future<Item = (), Error = io::Error>>), Error = io::Error> + Send + 'static

It probably could be rewritten in a more clear way but currently I don't see how.

[coveralls]

Coverage increased (+0.2%) to 55.857% when pulling 360f445 on swizard0:pull_req_heartbeat_impl_trait into d67f86e on sozu-proxy:master.

[Keruspe]

This is just me being nitpicky, but maybe rename "P" to "Pulse" and add the trait constraint ": Future + Send + 'static" for the struct and impl, mostly for "documentation" purpose.

Otherwise LGTM, thanks a lot!

[Keruspe]

Maybe add "+ Send + 'static" in the Heartbeat type param, just for consistency's sake?

[swizard0]

You're welcome.

I've made the fixes, currently except this:

and add the trait constraint ": Future + Send + 'static" for the struct and impl, mostly for "documentation" purpose

I'm not really sure that library users want to know this field type constraints because it is private and there is no way to get and use it for them.

Perhaps this unnecessary information only distracts the reader.