fix(client): try to reuse connections when pool checkout wins

If a checkout wins, meaning an idle connection became available before
a connect future completed, instead of just dropping the connect future,
it spawns it into the background executor to allow being placed into
the pool on completion.

src/common/mod.rs

@@ -1,8 +1,10 @@
 mod buf;
 mod exec;
 pub(crate) mod io;
+mod lazy;
 mod never;
 
 pub(crate) use self::buf::StaticBuf;
 pub(crate) use self::exec::Exec;
+pub(crate) use self::lazy::lazy;
 pub use self::never::Never;