The headers table from client to server
and the one from server to client
may have different maximum sizes
(even if both endpoints have to keep both tables)
This parameter is NULL or the pointer to the previous state
for the previous protocol in the case of a protocol change,
for instance from HTTP1 to HTTP2
This way, the new protocol can use the old protocol context.
For instance, HTTP2 mimicks the HTTP1 request, to have a HTTP2
transaction with both request and response
To signal incomplete data, we must return the number of
consumed bytes. When we get a banner and some records, we have
to take into account the number of bytes already consumed by
the banner parsing before reaching an incomplete record.