Resumption of partial transfers

[crazyscot]

If a large file has been part transferred, it would be useful if we could skip sending the bytes we already have.

• How robust a check do we need? Is file size enough?
• If the a copy was interrupted, there could be corruption (an incomplete block). We would ideally detect and work around that. (Possible future direction: full rsync protocol?)

Describe the solution you'd like
--resume CLI option, automatically detects part transfers and does the appropriate thing.
Optionally make resume mode a configuration option.