CAUTION! This version is not fool proof yet. If you purge data of an OSD by
overwriting the disk with zeroes and restart it then the same data will also
be removed from other replicas :-).
I plan to add protection from this situation before merging it into master.
The idea is to make each OSD store a random "cookie" on disk and remove itself
from history automatically if the cookie doesn't match.
Previously OSDs could commit deletes before writes during recovery or rebalance
in the "lazy fsync" (immediate_commit=off) mode which could result in lost objects
Vitaliy Filippov