* this class separates _allocations_ from _streams_. An allocation is created by a call, used for
 * one or more streams, and then released. An allocated connection won't be stolen by other calls
 * while a redirect or authorization challenge is being handled.
 *
 * When the maximum concurrent streams limit is reduced, some allocations will be rescinded.

 *  * http://host
 *
 * Both the unnecessary port specification (`:80`) and the absent trailing slash (`/`) cause URI to
 * bucket the two URLs separately. This harms URI's usefulness in collections. Any application that
 * stores information-per-URL will need to either canonicalize manually, or suffer unnecessary
 * redundancy for such URLs.
 *
 * Because they don't attempt canonical form, these classes are surprisingly difficult to use