net: lib: http_server: Implement proper CONTINUATION frame processing

CONTINUATION frames are tricky, because individual header fields can be
split between HEADERS frame and CONTINUATION frame, or two CONTINUATION
frames. Therefore, some extra logic is needed when header parsing
returns -EAGAIN, as we may need to remove the CONTINUATION frame header
from the stream before proceeding with headers parsing.

This commit implements the above logic and additionally adds more checks
to detect when CONTINUATION frame is expected. Not receiving a
CONTINUATION frame when expect should be treated as a protocol error.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>