return timestamp in get_order_book api call

[oxarbitrage]

To get the timestamp of an order book the developer will have to make a separated query to get the head_block_time. This can be a different time than the order book he/she is getting.

A timestamp field can be added among the bids and the asks with timestamp at the moment of pulling the order book.

Coming from bitshares/bitshares-explorer-api#64 (comment)

[abitmore]

Please be aware that

1. order book of normal API nodes usually contains unconfirmed data due to pending transactions, thus it can change when being included in a block;
2. even if it's included in a block, before the block become irreversible, the state can change due to chain reorganization;
3. order book of delayed_node usually doesn't make sense for traders.

So a timestamp especially head_block_time doesn't really make sense. IMHO you can just return a local time.

[oxarbitrage]

Makes sense to me.