implement zip-0317 fee calculation for ZEC

[cipig]

ZEC switched to a new fee system:
https://zips.z.cash/zip-0317
https://forum.zcashcommunity.com/t/understanding-why-transactions-are-taking-longer-to-be-mined-and-what-is-going-on-with-the-mempool/45399

the rule is basically this:
The new default fee is then 5000 zatoshis per logical action, with a minimum of 10000 zatoshis.
every input is a "logical action"

in coins file ZEC has "txfee": 10000, which covers the minimum, but each input needs 5000 sats additionally.
if fee is too low, tx is rejected with the transaction was rejected by network rules.\\n\\n66: tx unpaid action limit exceeded: 8 action(s) exceeds limit of 0

will set txfee to 100000 in coins, so that it's enough for txes with up to 20 inputs, but that's just a workaround till we have a better solution