MINOR: h3: support HTTP request framing state

MINOR: h3: support HTTP request framing state

Store the current step of HTTP message in h3s stream. This reports if we
are in the parsing of headers, content or trailers section. A new enum
h3s_st_req is defined for this.

This field is stored in h3s struct but only used for request stream. It
is left undefined for other streams (control or QPACK streams).

h3_is_frame_valid() has been extended to take into account this state
information. A connection error H3_FRAME_UNEXPECTED is reported if an
invalid frame according to the current state is received; for example a
DATA frame at the beginning of a stream.