Fix the credential binary to handle transactions to sleep when in non-inProgress epochs

[benedettadavico]

Currently fails with
thread 'main' panicked at 'attempt to subtract with overflow', clients/credential/src/main.rs:55:36
Possibly an issue around the time here, as epochs might take longer than 20 seconds to advance to next one (once the expected time has been reached)
let secs_until_final = epoch.final_timestamp_secs() + 20 - current_timestamp_secs;