libwebsockets

http: add RFC7231 date and time helpers and retry-after handling

Teach lws how to deal with date: and retry-after:

Add quick selftest into apt-test-lws_tokenize

Expand lws_retry_sul_schedule_retry_wsi() to check for retry_after and
increase the backoff if a larger one found.

Finally, change SS h1 protocol to handle 503 + retry-after: as a
failure, and apply any increased backoff from retry-after
automatically.