Partially removes "current frame" state from TemporalLayers, aiming to
tie more state to the frame config being encoded. This is necessary for
having several outstanding frames being encoded.
Also renames several structs, since TemporalReferences contains more
than references it's renamed TemporalLayers::FrameConfig.
BUG=chromium:702017, webrtc:7349
R=sprang@webrtc.org
Review-Url: https://codereview.webrtc.org/2853073004
Cr-Commit-Position: refs/heads/master@{#18012}