Settlement code refactory

Follow-up of #2721.

Changes:

Rename functions has_settlement() and has_individual_settlement() to is_globally_settled() and is_individually_settled_to_fund() for intuition.
Store settled debt amount and collateral amount in the individual_settlement_debt and individual_settlement_fund fields in bitasset object instead of new data fields in limit order. Note: Due to this change, the 2 fields will become non-zero when individual settlement occurs and BSRM is individual_settlement_to_order. It may break some client applications which rely on the old logic "the 2 fields will be always zero if BSRM is individual_settlement_to_order".

bitshares / bitshares-core