SSL Analyzer: track connection direction by messages

This PR changes the way in which the SSL analyzer tracks the direction
of connections. So far, the SSL analyzer assumed that the originator of
a connection would send the client hello (and other associated
client-side events), and that the responder would be the SSL servers.

In some circumstances this is not true, and the initiator of a
connection is the server, with the responder being the client. So far
this confused some of the internal statekeeping logic and could lead to
mis-parsing of extensions.

This reversal of roles can happen in DTLS, if a connection uses STUN -
and potentially in some StartTLS protocols.

This PR tracks the direction of a TLS connection using the hello
request, client hello and server hello handshake messages. Furthermore,
it changes the SSL events from providing is_orig to providing is_client,
where is_client is true for the client_side of a connection. Since the
argument positioning in the event has not changed, old scripts will
continue to work seamlessly - the new semantics are what everyone
writing SSL scripts will have expected in any case.

There is a new event that is raised when a connection is flipped. A
weird is raised if a flip happens repeatedly.

Addresses GH-2198.

scripts/policy/protocols/ssl/validate-ocsp.zeek

@@ -31,7 +31,7 @@ export {
 # certificate chain is seen.
 global recently_ocsp_validated: table[string] of string = table() &read_expire=5mins;
 
-event ssl_stapled_ocsp(c: connection, is_orig: bool, response: string) &priority=3
+event ssl_stapled_ocsp(c: connection, is_client: bool, response: string) &priority=3
 	{
 	c$ssl$ocsp_response = response;
 	}