issue-#170

[andrewnguyen22]

Describe the bug

The order book structure is stored inefficiently and needs to be redone before mainnet:

Specifically

The structure of the order books as a list vs individual unique orders is estimated to have an unacceptable space complexity

Expected behavior

Change the id of the sell order

message SellOrder {
  // id: the unique identifier of the order
  uint64 Id = 1;
...

to unique deterministic bytes (the hash of the transaction hash that opened the order)

message SellOrder {
  // id: the unique identifier of the order should 
  bytes Id = 1; // transaction hash
}

And change the order book key to a prefix.

func KeyForOrderBook(id uint64) []byte  { return lib.JoinLenPrefix(orderBookPrefix, formatUint64(id)) }

to

func PrefixForOrderBook(chainId uint64) []byte  { return lib.JoinLenPrefix(orderBookPrefix, formatUint64(chainId)) }

and add

func KeyForOrder(chainId uint64, id []byte) []byte  { return lib.JoinLenPrefix(orderBookPrefix, formatUint64(id)) }

then finally change the order book logic to utilize the new design, where orders are stored individually (not a slice) and order books are created dynamically by iterating over the prefix for the committee.

Additional context

This is technically a consensus breaking issue, but because there's been no orders so far -- we should be fine to do this change on the fly.

closes #170

[rem1niscence]

approved! even though a lot was touched, seems like the same final behavior is kept and is more like a big refactor.