Staking invariant: Check to make sure power store key aligns with validator power

[sunnya97]

Replacement to #2620 as checking that validator power is >0 is a somewhat self-referential check as you have to filter by the validators whose power >0 first.

Sub-components:

• delete old invariant check power greater than 0
• add the new invariant check, that the PowerIndexCheck is correct

Arose out of discussions with @jaekwon and @rigelrozanski

[alexanderbez]

Should this be closed via #2671 or are we waiting until it's in develop? I think this should be closed.