Make Idle Timeout configurable on Hyper 1.0

In #4282/#4284, the bug where Hyper 1.0 had no idle timeout was fixed. However, it still is impossible to configure the idle timeout.

We should make the idle timeout a configurable setting on the builder. I think its reasonable to assume that any pool that will be backing Hyper will always have this setting so it isn't much of a semver hazard.