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)
Notably:
- fix the `delete op` inside lambda callback crash (it frees the lambda itself
which results in use-after-free with g++)
- fix stop_client() reenterability
- fix a bug in the blockstore layer which resulted in always returning version=0
for zero-length reads
- change error codes for blockstore_stabilize
Vitaliy Filippov