Add option for MPA owners to disable collateral bidding

PR for #2281.

Add new option for MPA owners to disable collateral bidding.

Specifications:

new disable_collateral_bidding bit in asset flags and issuers_permissions
- unable to set before hard fork
- unable to set on UIAs, but can be set on PMs (although useless)
- common logic about issuer permissions (if permission is forfeited, unable to update flag, unable to enable permission if supply is non-zero, etc)
if set in flags, unable to bid collateral
when newly configured via asset_update_operation, cancel all collateral bids
at hard fork time, cancel all collateral bids of asset which already disabled collateral bidding (due to old bug)

bitshares / bitshares-core