Really unique session ID - PID + connection timestamp?

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:

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