[endurance] keep endurance dir in place when packing snapshots

Let's only move the snapshots to $work_dir as later we'll want to store
current session's total snapshot count in a file in $ENDURANCE_DIR, and
that file has to be preserved between snapshot packs, as long as the
session lasts.
[endurance] require at least 2 snapshots in a session

Sessions having only one snapshot are useless, and thus eliminate them.
[endurance] initialize $snapshots_in_session with $snapshots_not_uploaded

This is for the transition period between old and new endurance-collect
script, where $SNAPSHOT_COUNT_FILE didn't exist.
Incremented version