Make deletions instantly stable

"2-phase" (write->stabilize) process is pointless for deletions because it
doesn't protect us from incomplete objects. This happens because it removes
the version information from metadata after stabilization. Deletions require
"3-phase" process with a potentially very long 3rd phase.

So, deletions will be allowed to generate degraded and incomplete objects,
and for it to not affect users' ability to delete something, the cluster
will allow to delete whole inodes while storing a list of them in etcd.
Proper TRIM will be impossible until the implementation of the aforementioned
"3-phase" process, though.

By the way, this change also fixes a possible write stall after rebalancing
which was caused by the lack of "stabilize delete" operations.

blockstore_sync.cpp

@@ -275,7 +275,16 @@ void blockstore_impl_t::ack_one_sync(blockstore_op_t *op)
 #endif
         auto & unstab = unstable_writes[it->oid];
         unstab = unstab < it->version ? it->version : unstab;
-        dirty_db[*it].state = dirty_db[*it].state == ST_DEL_WRITTEN ? ST_DEL_SYNCED : ST_J_SYNCED;
+        if (dirty_db[*it].state == ST_DEL_WRITTEN)
+        {
+            dirty_db[*it].state = ST_DEL_SYNCED;
+            // Deletions are treated as immediately stable
+            mark_stable(*it);
+        }
+        else /* == ST_J_WRITTEN */
+        {
+            dirty_db[*it].state = ST_J_SYNCED;
+        }
     }
     in_progress_syncs.erase(PRIV(op)->in_progress_ptr);
     op->retval = 0;