balance: remove Pool (#658)

Per #456, there are a number of issues with the `balance::Pool` API that
limit its usability, and it isn't widely used. In the discussion on that
issue, we agreed that it should probably just be removed in 0.5 --- it
can be replaced with something more useful later.

This branch removes `balance::Pool`.

CLoses #456.

tower/src/balance/mod.rs

@@ -5,21 +5,14 @@
 //! Otherwise, clients could see spikes in latency if their request goes to a particularly loaded
 //! service, even when spare capacity is available to handle that request elsewhere.
 //!
-//! This module provides two pieces of middleware that helps with this type of load balancing:
-//!
-//! First, [`p2c`] implements the "[Power of Two Random Choices]" algorithm, a simple but robust
-//! technique for spreading load across services with only inexact load measurements. Use this if
-//! the set of available services is not within your control, and you simply want to spread load
-//! among that set of services.
+//! This module provides the [`p2c`] middleware, which implements the "[Power of
+//! Two Random Choices]" algorithm. This is a simple but robust  technique for
+//! spreading load across services with only inexact load measurements. Use this
+//! if the set of available services is not within your control, and you simply
+//! want to spread load among that set of services.
 //!
 //! [Power of Two Random Choices]: http://www.eecs.harvard.edu/~michaelm/postscripts/handbook2001.pdf
 //!
-//! Second, [`pool`] implements a dynamically sized pool of services. It estimates the overall
-//! current load by tracking successful and unsuccessful calls to [`poll_ready`], and uses an
-//! exponentially weighted moving average to add (using [`MakeService`]) or remove (by dropping)
-//! services in response to increases or decreases in load. Use this if you are able to
-//! dynamically add more service endpoints to the system to handle added load.
-//!
 //! # Examples
 //!
 //! ```rust
@@ -52,10 +45,6 @@
 //! }
 //! # }
 //! ```
-//!
-//! [`MakeService`]: crate::MakeService
-//! [`poll_ready`]: crate::Service::poll_ready
 
 pub mod error;
 pub mod p2c;
-pub mod pool;

tower/src/balance/p2c/service.rs

@@ -213,10 +213,6 @@ where
         let (_, svc) = self.services.get_ready_index(index).expect("invalid index");
         svc.load()
     }
-
-    pub(crate) fn discover_mut(&mut self) -> &mut D {
-        &mut self.discover
-    }
 }
 
 impl<D, Req> Service<Req> for Balance<D, Req>