[HARDFORK] Feature/asset disable limit order create flag

[xeroc]

This pull request introduces code that allows to set a new flag for an asset. This flag only_issuer_limit_orders_allowed 0x200 can be used to prevent other market participants from placing orders in the market. The feature is particularly useful in case of an ICO with multiple stages where only the issue should be able to sell tokens and prevent previous participants from making instant profits.

A corresponding BSIP will follow (paging @Dimfred)

[xeroc]

BSIP: https://github.com/bitshares/bsips/issues/148

As can be seen from the BSIP, an open question is if it makes sense to use a simple binary toggle, or maybe instead introduce another whitelist to the asset.

[pmconrad]

Proper procedure is

1. Write a BSIP
2. Once it's deemed complete: have it approved
3. Once it's been approved: implement (you may implement it at your own risk before approval of course)
4. Once implementation is complete: open PR

[xeroc]

1. Write a BSIP

2. Once it's deemed complete: have it approved

3. Once it's been approved: implement (you may implement it at your own risk before approval of course)

4. Once implementation is _complete_: open PR

We wanted to shortcut internally and asked our developers to implement this as part of a learning process. It was lying around for many months and I thought we should rather shoot this upstream than forget about it ... Now that we have more structure in place internally, we hope to follow procedure better in the future. Sorry for the inconvenience.

[oxarbitrage]

If going forward with this implementation the code need to be against hardfork branch.

[oxarbitrage]

Also, as usual with hardfork changes, need guards to make sure the flag can not be used before date.