Record a session replay chunk

What the replay recorder (/api/r.js) sends: one chunk of rrweb events of the visitor's current session, gzipped (Content-Encoding: gzip) or, for the last flush of a page, plain JSON as text/plain. No credential: the visitor is derived from the request, as for events. 204 whether the chunk was stored or dropped (replay off, no live session, a bot, the session's caps reached); 400 for a malformed chunk, 404 for an unknown site, 413 past the size limits, 415 for another content type, 429 past the per-client rate limit, 503 when busy or storage is unavailable.