Experimental: Handle degraded deletions by comparing object versions with epochs

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.

src/etcd_state_client.h

@@ -33,7 +33,7 @@ struct pg_config_t
     bool exists;
     osd_num_t primary;
     std::vector<osd_num_t> target_set;
-    std::vector<std::vector<osd_num_t>> target_history;
+    std::vector<pg_history_set_t> target_history;
     std::vector<osd_num_t> all_peers;
     bool pause;
     osd_num_t cur_primary;