timeclock: allow multiple open sessions

[simonmichael]

hledger's timeclock format doesn't allow clocking in to multiple named sessions concurrently, at least not within a single timeclock file. I think Ledger does allow this. We'd like to be able to freely clock in and out of different named sessions, tracking them individually. This means clock outs need to be matched with the right open sessions.

[simonmichael]

Related discussion on #2142