[token-server] Implement ecash-lib for tx building

Summary:
Start building rewards txs using ecash-lib instead of utxo-lib

Doing this now because looking to add an XEC-only airdrop for new users.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D16555