Refactor server module.

In the interest of the user's attention, some ancillary APIs have been
moved to new submodules:

- server::limits contains what was previously called Throttler and
  ChannelFilter. Both of those names were very generic, when the methods
  applied by these types were very specific (and also simplistic). Renames
  have occurred:
  - ThrottlerStream => MaxRequestsPerChannel
  - Throttler => MaxRequests
  - ChannelFilter => MaxChannelsPerKey
- server::incoming contains the Incoming trait.
- server::tokio contains the tokio-specific helper types.

The 5 structs and 1 enum remaining in the base server module are all
core to the functioning of the server.

example-service/src/server.rs

@@ -17,7 +17,7 @@ use std::{
 };
 use tarpc::{
     context,
-    server::{self, Channel, Incoming},
+    server::{self, incoming::Incoming, Channel},
     tokio_serde::formats::Json,
 };
 use tokio::time;