Fixed bugs that caused ensure_shares to not work properly. Mainly:
- Comparison between db object and hash string was causing
ensure_shares to run every time. Fixed by reading property
'info_hash' of db object.
- Driver implementation of ensure_shares would raise an
exception in case of returning None. Fixed by adding
'or {}' to method invocation.
- Missing share_servers parameter for each share passed
to ensure_shares. Fixed by converting share_instance
entities to dictionaries before passing to drivers.
Closes-bug: #1771866
Closes-bug: #1772644
Closes-bug: #1772647
Change-Id: Iac969e9cac6cea46deb12f5d8423be592cfeb72a
