Generalize TcpServer::serve's service a bit

[alexcrichton]

The services accepted by TcpServer::serve wire up the service's
request/response types the underlying request/response types of the BindServer
implementation. These types tend to be what we want for the simple case (non
streaming), but for the streaming case they have the
tokio_proto::streaming::Message type wired in.

The Message type typically isn't what libraries want as the request/response
types for their services, so this PR generalizes to use From and Into to
ensure that custom types can still be used so long as they can be converted.

I think this sould be backwards compatible as well (not causing inference
regressions), hopefully at least.

[alexcrichton]

The generics got a little unwieldy here, but I wasn't sure why all the 'static bounds popped up...

[aajtodd]

Might be good to have an example of this (even if it's just a compile test).

[alexcrichton]

Yeah the intention was that libraries would implement From<Message> and Into<Message> for their request/response types and could then use this builder, but unfortunately the motivation here (Hyper) still can't use this because it needs access to the remote address of the underlying TCP stream. As a result I'm not sure that we'd want to pursue this just yet, but curious on others' thoughts still.

[aturon]

LGTM.

[carllerche]

@alexcrichton suggested holding off on this for now.

[alexcrichton]

So over our lunch discussion today I'm actually inclined to go ahead and merge, @aturon @carllerche sound ok?

[aturon]

Yep, I think this is a step in the right direction.

[carllerche]

Seems fine w/ me

[alexcrichton]

Alright let's do this!