Make explicit decision on removing or keeping 0 int's in state for total escrow amount

[colin-axner]

Summary

#3019 added a separate tracking amount in state for the total IBC'd out for a chain. One slight concern is handling of 0 amount escrow balances. Two issues arise:

• GetTotalEscrowForDenom returns a zero coins slice for escrows which have never been referenced to ease caller logic (not ideal)
• 0 int's exist in state while the bank keeper prunes zero balances (also not ideal to have inconsistency)

We should make an explicit decision on how we want to handle this situation, so we can ensure there are no pitfalls and that it is well documented

Given two problematic scenario's, I think I'm leaning towards adjusting the logic to:

• require caller to handle initializing empty escrow accounts
• prune zero ints (match bank behaviour)

I guess the idea is that the escrow function needs to handle empty escrows and unescrow needs to handle pruning zero amounts.

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For Admin Use

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[crodriguezvega]

Closed by #3585