Really unique session ID - PID + connection timestamp?

Dear Everybody!

In MS we had a "persistent lock" structure and method.
This over transactions because based on real records in a real table with
pri key (tablename + id).

For garbaging we had a special session info.
In MS the session id is smallint, so it can repeats after server restarts,
but my coll. found a "session creation timestamp".
This is a key which unique.
With this we can check for died sessions and we can clean their records.

We want create same mechanism.
I know there are adv. locks in PG, but I want to use session id.

This could be:
pg_backend_pid()

May pid repeats.
Where I can get timestamp or some other unique data with I can create a
combined primary key?

Thanks for your help!

dd