More stable funding rate estimates

[TomMcL]

Currently the funding rate displayed in Console, an estimate derived from core, varies significantly across time as the time moves closer to the funding payment time. This is likely because the funding rate displayed is the funding rate for the period from the last payment until now, rather than extrapolated across the whole window.

Investigate:

1. Exactly how other exchanges display this, we should do similar to ensure people can compare
2. How to do that/where changes need to be made

[davidsiska-vega]

I believe that with the L2 more frequent data sourcing and mark price improvements we are ok with funding as-is. Closing for now (happy to re-open if issues re-surface).

[davidsiska-vega]

I believe that with the L2 more frequent data sourcing and mark price improvements we are ok with funding as-is. Closing for now (happy to re-open if issues re-surface).

Actually it's about the estimate so it's just a question of defining behaviour and getting core to produce the right number.