Check for interrupted send session, then resume

[tasket]

Check metadata for '-tmp' when starting a send, then find which chunks were sent and continue sending from that point.

Associated changes:

• class Volume initial detection
• prepare_snapshots() with special condition for monitor_only mode
• monitor_send() and send_volume()
• new function to scan/sanitize untrusted destination directory... maybe simple counter on destination that returns last chunk
• offer user to verify the session as a unit when resumed send is completed

[tasket]

Using a checkpoint file in /var that is repeatedly updated during a send could make this possible. The update interval could be based on 'bcount', using a large value possibly >100MB to be worthwhile.

If the checkpoint exists both locally and on dest with the same value, then check that chunk files exist & are non-zero, and resume send from that point.