GH-1119: add base/protcols/conn/removal-hooks.zeek

This adds two new functions: `Conn::register_removal_hook()` and
`Conn::unregister_removal_hook()` for registering a hook function to be
called back during `connection_state_remove`.  The benefit of using hook
callback approach is better scalability: the overhead of unrelated
protocols having to dispatch no-op `connection_state_remove` handlers is
avoided.

scripts/base/protocols/krb/files.zeek

@@ -118,7 +118,7 @@ event krb_tgs_response(c: connection, msg: KDC_Response)
 	fill_in_subjects(c);
 	}
 
-event connection_state_remove(c: connection)
+hook finalize_krb(c: connection) &priority=+5
 	{
 	fill_in_subjects(c);
 	}