New BSIP: Conditional order - stop-limit, OSO, etc.

## Motivation
There are multiple order types that makes trading much more convenient that are available on other exchanges and not available on BitShares.
They usually are conditional and are based on the price.
There are multiple examples of such order types: Stop-loss, Take profit, Stop-limit, OSO, OCO.
We believe that adding this highly convenient trading tools would make BitShares much more attractive to many traders.

## Abstract
A topic of Stop or conditional orders has appeared a few times in the forums and on the github.

A stop-limit order is an order to place a regular buy or sell order (also known as a "limit order") when the highest bid or lowest ask reaches a specified price, known as the "stop." This can be helpful for protecting profits or minimizing losses.
Traders can create stop-limit orders with specified limit and stop prices.

Our understanding is that exposing the conditional orders to public might jeopardize trading for the the trader who put the order.
**Hence, the biggest challenge is to hide this kind of orders from everybody except the trader and the blockchain**
And we think that solving this technical challenge is quintessential for this BSIP.

Please let us know your thoughts,
OpenLedger team