Implement filtered buy orders

[ryanchristo]

Summary

Currently MsgBuy implements a buy order selection with the option to create a direct buy order using sell_order_id. Once we have filters defined for baskets, we can enable filtered buy orders and implement buy order processing.

    // Selection defines a buy order selection.
    message Selection {

      // sum defines the type of selection.
      oneof sum {
        // sell_order_id is the sell order ID against which the buyer is trying to buy.
        // When sell_order_id is set, this is known as a direct buy order because it
        // is placed directly against a specific sell order.
        uint64 sell_order_id = 1;

        // TODO: once we have filters defined for baskets, we can enable filtered
        // buy orders which will involve full double-sided batch order matching
        //
        // filter selects credits to buy based upon the specified filter criteria.
        // Filter filter = 2;
      }
    }

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[ryanchristo]

We will need to take second look at the message fields for the BuyOrderCreated event. See #613 (comment).

[clevinson]

Should this be included in the next upgrade? It seems like the feature is well enough specified where we could get support for non-direct buy orders in this same release.

Thoughts @aaronc @dtkong ?