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.