perf: modify DelegatorSharesInvariant for better performance

[catShaark]

Description

Closes: #XXXX

The code in DelegatorSharesInvariant is inefficient as it uses k.GetValidatorDelegations for each validator in valset, k.GetValidatorDelegations iterates through all the delegations and return all delegations that has validator address equal to the validator address passed in. This mean we have len(valset) number of all delegations iterations.

Proposal

Change the code in DelegatorSharesInvariant so that it iterate all delegations once, reduce genesis import time quite a bit.

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[catShaark]

I did profiling on importing genesis. I imported a 1.8G genesis file ( exported from an osmosis mainet node ), it takes 241 sec while the DelegatorSharesInvariant invariant alone takes 85 sec.

[alexanderbez]

LGTM. @ValarDragon mind taking a quick look?

[ValarDragon]

Logic lgtm! Needs comment specifying that were iterating this way / what's happening in these lines