Make server methods more composable.

-- Connection Limits

The problem with having ConnectionFilter default-enabled is elaborated on in https://github.com/google/tarpc/issues/217. The gist of it is not all servers want a policy based on `SocketAddr`. This PR allows customizing the behavior of ConnectionFilter, at the cost of not having it enabled by default. However, enabling it is as simple as one line:

incoming.max_channels_per_key(10, ip_addr)

The second argument is a key function that takes the user-chosen transport and returns some hashable, equatable, cloneable key. In the above example, it returns an `IpAddr`.

This also allows the `Transport` trait to have the addr fns removed, which means it has become simply an alias for `Stream + Sink`.

-- Per-Channel Request Throttling

With respect to Channel's throttling behavior, the same argument applies. There isn't a one size fits all solution to throttling requests, and the policy applied by tarpc is just one of potentially many solutions. As such, `Channel` is now a trait that offers a few combinators, one of which is throttling:

channel.max_concurrent_requests(10).respond_with(serve(Server))

This functionality is also available on the existing `Handler` trait, which applies it to all incoming channels and can be used in tandem with connection limits:

incoming
    .max_channels_per_key(10, ip_addr)
    .max_concurrent_requests_per_channel(10).respond_with(serve(Server))

-- Global Request Throttling

I've entirely removed the overall request limit enforced across all channels. This functionality is easily gotten back via [`StreamExt::buffer_unordered`](https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.1/futures/stream/trait.StreamExt.html#method.buffer_unordered), with the difference being that the previous behavior allowed you to spawn channels onto different threads, whereas `buffer_unordered ` means the `Channels` are handled on a single thread (the per-request handlers are still spawned). Considering the existing options, I don't believe that the benefit provided by this functionality held its own.

example-service/src/lib.rs

@@ -10,5 +10,9 @@
 // It defines one RPC, hello, which takes one arg, name, and returns a String.
 tarpc::service! {
     /// Returns a greeting for name.
-    rpc hello(name: String) -> String;
+    rpc hello(#[serde(default = "default_name")] name: String) -> String;
+}
+
+fn default_name() -> String {
+    "DefaultName".into()
 }