Taker rewards capped by trading fees reward scheme

[davidsiska-vega]

Consider adding a taker reward which caps the reward by trading fees paid.

At the moment the taker rewards are roughly calculated as follows: the scheme specifies the fee type which makes you eligible (taker / maker)

1. there is an amount to be distributed
2. there are keys that have paid the relevant fees; we calculate the fraction of 1 of relevant fees each eligible key paid/received.
3. each key then gets fraction times the total.

In particular it means that e.g. if the reward pot is 500 and there is only one trader who did one trade they get all 500.

As a separate scheme we want to add the following

1. there is an amount to be distributed
2. there are keys that have paid the relevant fees; we calculate the fraction of 1 of relevant fees each eligible key paid/received.
3. each key then gets max(fraction times total, taker fees paid in epoch times conversion factor from quantum to reward asset).
4. whatever is left over stays in the reward pool for next epoch (if this is valid) or sent to the key that funded the pool or whatever makes most sense in the current design.