[BUG] BCH estimate tx size is much less than the final size , cause min relay fee issue

We are still having the min relay fee not met issue on tBCH, as a result of that the vault migration is not complete.

Here is what I am thinking:

1. The fee rate on BCH somehow is way too low , at the moment , it is about 1 sat ~3 sats per byte , other chains like LTC / BTC , it is way more than that , like 20 ~ 40 sats per byte
2. At the moment, before we sign a tx out , bifrost is trying to estimate the final tx size to decide how much gas to use, but the estimate size is way too low , on BCH right now the estimated size is about 514, however the final size is 1544

Possible Solutions

1. We set a min sats per byte for BCH , like 20? we don't spend less than 20 sats/byte, if we can do that , we won't have the min relay fee issue.
2. Jack up the estimate tx size , like multiple by 3

I prefer to solution 1 , just set a min fee rate , never spend lower than that