feat(pool): add a Cache pooling service (#227)

This creates a new `Cache` type that acts as a pool over an inner make-service. Asking the Cache to produce a service will either return a cached one, or create a new one with the inner maker. The returned service is `Cached<S>`, which knows to return the service to the cache upon Drop.

Closes hyperium/hyper#3954

Author: seanmonstar

Cargo.toml

@@ -76,7 +76,7 @@ full = [
 
 client = ["hyper/client", "tokio/net", "dep:tracing", "dep:futures-channel", "dep:tower-service"]
 client-legacy = ["client", "dep:socket2", "tokio/sync", "dep:libc", "dep:futures-util"]
-client-pool = []
+client-pool = ["dep:futures-util"]
 client-proxy = ["client", "dep:base64", "dep:ipnet", "dep:percent-encoding"]
 client-proxy-system = ["dep:system-configuration", "dep:windows-registry"]