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/ssh/main.zeek

@@ -247,7 +247,7 @@ event ssh_capabilities(c: connection, cookie: string, capabilities: Capabilities
 	                                 server_caps$server_host_key_algorithms);
 	}
 
-event connection_state_remove(c: connection)
+event successful_connection_remove(c: connection)
 	{
 	if ( c?$ssh && !c$ssh$logged )
 		{