[SON-HIVE Voting] Refactor SON object to support multiple vote_ids

The goal is to separate voting for Bitcoin and Hive SONs, so that they can act independently of each others. All code affected by this change should be refactored, in a way that enables that Bitcoin SON and Hive SON are considered separate entities.

Current:

   class son_object : public abstract_object<son_object>
   {
      public:
         static const uint8_t space_id = protocol_ids;
         static const uint8_t type_id  = son_object_type;

         account_id_type son_account;
         vote_id_type vote_id;
         uint64_t total_votes = 0;
         string url;
         vesting_balance_id_type deposit;
         public_key_type signing_key;
         vesting_balance_id_type pay_vb;
         son_statistics_id_type statistics;
         son_status status = son_status::inactive;
         flat_map<sidechain_type, string> sidechain_public_keys;

         void pay_son_fee(share_type pay, database& db);
         bool has_valid_config()const;
         bool has_valid_config(time_point_sec head_block_time)const;
   };

Wanted:

   class son_object : public abstract_object<son_object>
   {
      public:
         static const uint8_t space_id = protocol_ids;
         static const uint8_t type_id  = son_object_type;

         account_id_type son_account;
         vote_id_type vote_id_bitcoin;
         vote_id_type vote_id_hive;
         uint64_t total_votes = 0;
         string url;
         vesting_balance_id_type deposit;
         public_key_type signing_key;
         vesting_balance_id_type pay_vb;
         son_statistics_id_type statistics;
         son_status status = son_status::inactive;
         flat_map<sidechain_type, string> sidechain_public_keys;

         void pay_son_fee(share_type pay, database& db);
         bool has_valid_config()const;
         bool has_valid_config(time_point_sec head_block_time)const;
   };