python: added bulk_read_by_id() API

Also added alias from bulk_read() to bulk_read_by_name()
recovery: bulk read/write resulting diff's
recovery: replaced manual caching with backported lru_cache
recovery: optimize computing off diff for empty local or/and remote containers
recovery: fix variable shadowing in recover()
recovery: fixed --groups option
recovery: set default timeout to 3600
recovery: improved comments
recovery: simplify dnet_iterator_response_container_diff()
recovery: simplify flag handling in elliptics_create_session()
ext: eblob: threat old-format records same as non-existent.

This fixes bug on old-format/new-format data rewrite.
Also explicitly set err if not already set, just in case.
recovery: added Stats API
gitignore: added *.py[co]
recovery: formatting and comments
gitignore: sorted