This allows list responses to be unaffected by journal flushes, which, in turn,
fixes PG peering when a peer OSD is replaying journal and journal contains deletions

"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.

The stall occurred during fio Q=128 random write tests with low flusher_count (4).
It was caused by flushers being unable to flush the beginning of the journal
because it contained older writes to an object that also had writes in the very end
of the journal, after dirty_start.

(to fix use-after-free, too, but "delete this" was a time bomb anyway)

- Do not block flock() requests
- Fix stop_client(0) attempts leading to std::bad_function_call
- Fix degraded writes crashing due to an unset stripes[i].missing (at least with a missing parity device)
- Fix recovery B/W reporting

Instead of only removing versions with oid == X and version > Y it was
also removing the previous version in list (with the previous oid or
with version == Y)

Fixes the bug where two peers killed at once have lead to PG state PG_DEGRADED|PG_HAS_INCOMPLETE instead of PG_INCOMPLETE