snazzer-receive should somehow mark snapshots on the sending host as "sent"

[csirac2]

Idea: a snapshot /foo/.snapshotz/YYYY-MM-DDT000000Z, when sent, should have a file /foo/.snapshotz/.sent/YYYY-MM-DDT000000Z with the receiving hostname + datetime appended. Basically, we reinvent the archive bit...

... this would become useful in a future iteration of snazzer which could be smarter about pruning; such as pruning based on disk free or warning/erroring on pruning of snapshots which haven't been sent elsewhere yet.