GH-646: add new "successful_connection_remove" event

And switch Zeek's base scripts over to using it in place of
"connection_state_remove".  The difference between the two is
that "connection_state_remove" is raised for all events while
"successful_connection_remove" excludes TCP connections that were never
established (just SYN packets).  There can be performance benefits
to this change for some use-cases.

There's also a new event called ``connection_successful`` and a new
``connection`` record field named "successful" to help indicate this new
property of connections.

scripts/base/protocols/socks/main.zeek

@@ -111,7 +111,7 @@ event socks_login_userpass_reply(c: connection, code: count) &priority=5
 	c$socks$status = v5_status[code];
 	}
 
-event connection_state_remove(c: connection)
+event successful_connection_remove(c: connection)
 	{
 	# This will handle the case where the analyzer failed in some way and was
 	# removed.  We probably  don't want to log these connections.