possibility of sync from two kind of sources

now it is possible to sync hivemind from node or from
postgres database prepared by a node.

Still it is required to have connection with the node because some
meta information is required delivered by the api calls:
database_api.get_dynamic_global_properties
database_api.get_feed_history
condenser_api.get_order_book

To sync with db add switches:
--hived-database-url=postgresql://user:pass@127.0.0.1/database
--max-batch=<number of blocks for each sql query>