swiftrepl: add a -o (once) option

Add a -o (once) option that doesn't loop over containers forever but
finishes up and moves to the next container when it's done. Especially
helpful with sync_deletes().

Also, join all threads in the main thread instead of while True:
sleep(10). This in combination with the above means that swiftrepl
actually exits when done with all containers.

Change-Id: I265e81c341568373d952e69804cb7611928bd6d0
swiftrepl: add a much faster sync_deletes() method

Instead of looping through all files and issuing HEADs for each of them,
compare container listing pages between source & destination and HEAD
only on the files existing on destination but not on source.

This cuts down runtime from weeks/months to ~4 hours for commons
containers.

Change-Id: Ibcb532a38605a6cf7006564ef6e35686404ce5ff