Optimize iteration in asset_update_bitasset_evaluator

[abitmore]

As described in #932 (comment),

For better performance, we can add a "by_backing_asset" index for asset_bitasset_data_object so can iterate through it here, rather then iterating through all bitAssets.

Related code is:

bitshares-core/libraries/chain/asset_evaluator.cpp

Lines 380 to 387 in 0c75388

	// loop through all assets that have this asset as a backing asset
	const auto& idx = d.get_index_type<asset_index>().indices().get<by_type>();
	for( auto itr = idx.lower_bound(true); itr != idx.end(); ++itr )
	{
	const auto& child = *itr;
	if ( child.bitasset_data(d).options.short_backing_asset == op.asset_to_update )
	{

[jmjatlanta]

Fixed by PR# /pull/1019