MINOR: stream: Add http-buffer-request option in the waiting entities

MINOR: stream: Add http-buffer-request option in the waiting entities

When http-buffer-request option is set on a proxy, the processing will be
paused to wait the full request payload or a full buffer. So it is an entity
that block the processing, just like a rule or a filter that yields. So now,
it is reported as a waiting entity if an error or a timeout occurred.

To do so, an stream entity type is added for this option. There is no
pointer. And "waiting_entity" sample fetch returns the option name.