Bitmex execution reports needs to be resubmitted as a PR

[badgerwithagun]

#191 originally contained Bitmex execution report support submitted by @KapitalTrading. It had heavy overlap with some other urgent PRs, so maintaining a "roll up" of these PRs (which I did in #191) was the only practical way of managing the conflicts during the months when PRs weren't getting applied.

Now, however, things are moving and we are slowly clearing the backlog of urgent PRs.

This Bitmex functionality was mixed in with various other useful performance tweaks and bug fixes which we wanted to get merged urgently. However, we have been short of Bitmex testers/reviewers so this functionality was extracted from that PR with the intention of reapplying it later:

Reversal commit from the #191 branch: 55513ff

I have a task to resubmit a PR with this functionality re-applied. It will be broken though (the XChange 4.3.11 upgrade broke it) so I'll need to re-assign it to someone else to finish off (I don't use Bitmex).

This is just a placeholder so I remember to do it.

[badgerwithagun]

Raised #244