Please note, changesets from previous open PRs have crept in.

A. Draft PR for soliciting feedback on completed 1..4 issue items.

1. Make the gas associated with all market messages identical, so that MsgCancelLimitOrder potentially carries same cost as MsgCancelReplaceLimitOrder.
2. New orders that add liquidity should receive a gas rebate proportional to how much of the order was filled already (0% fill means 100% rebate):
   

   em-ledger/x/market/keeper/keeper.go

   Line 334 in 12b0302

   if addToBook {

3. Canceled orders always pay full gas.
4. Replaced orders are only eligible for the gas rebate in 2. if they are replacing an order older than, say, 5 minutes. This requires Add creation timestamp to orders #34 to work.

B. This PR does not address issue section: Additional considerations.

Suggestions below

It must not be possible to exploit the rebate mechanism in a way where it becomes cheaper (or even free) to remove liquidity, i.e. crafting an order to get almost filled but adding a small amount to the book should not result in a rebate.
It must not be possible to spam the chain with useless transactions. Only behaviour that is net adding liquidity

Half baked ideas

1. Make Liquidity Fee a minimal dust amount i.e. 250 resulting in free single transactions, but costly for sending a large number of transactions. A known practice.
2. Do not rebate more than x number of free liquid orders per account at any time. Not a great solution in isolation.
3. Do not rebate more than x number of duplicate instruments orders per account at any time.
4. Do not rebate any duplicate instrument order that does not offer qty/pricing spread differential above a threshold i.e. the moving average of the last x booked instruments orders + 15%.

C. Three Market parameters have been added

• May leave them like that (beginblock)
• Add them to genesis
• Const them (what if we change them?)

message TxParams {
  // default fee for a market transaction.
  uint64 trx_fee = 1;

  // Valid liquidity adding orders are free(0) or set to this minimal fee
  uint64 liquid_trx_fee = 2;

  // Minutes interval for eligible replacing transactions to receive a rebate.
  // For a rebate to apply, the replacing transaction should occur these minutes
  // after the signer's original trx.
  int64 liquidity_rebate_minutes_span = 3;
}