New state providers specific to account metadata and current account balances

Basing on positive results of research described in the issue #75 (closed) add new builtin state providers for:

• account metadata
• account current balances (specific to all tokens: hive, hbd, vests). It should include also saving balance and reward balance - generaly all data required by database_api::get_account API (or similar). It is possible that we need to extend intearfce between hived and sql_serializer to simplify balance collection (and eliminate a need to track operations separately at state-provider side). If above data will be supplemented, it will be possible to move account metadata collection and query from hived into HAF based application.