Consider using a more realistic fee rate (not fixed 20 sats/vB) to calculate depositor fee

[ws4charlie]

from Panruo:
our current bitocin accounting is a bit off--TSS will be running deficit
reason is for each outbound btc tx, both depositor and withdrawal pay ZRC20 btc for gas fee.
The depositor pays 20sats/vB, withdrawer pays the 2x current price (96sats right now).
The TSS pays 2x current price for the whole vB (inputs + output)
for a 10inputs + 3outputs typical tx, TSS is losing money, as TSS pays 2x current priced for all inputs while depositors only pay 20sats/vb

from Charlie:
The withdrawer fee is fixed and charged as 254vB * watchedGasPrice in zetacore
The depositor fee is also fixed as 68vB * 20sats/vB per deposit.

For each N-input-3-output outbound transaction:
withdrawer pays 254vB * watchedGasPrice
depositors pays 68vB * (N-1) * 20sat/vB
tss signer pays txSizeVb * watchedGasPrice

When watchedGasPrice > 20sat/vB
tss signer lose: (txSizeVb - 254 - 68*(N-1)) * (watchedGasPrice - 20sat/vB)