[ecash-coinselect] Simplify to match functionality of coinselect library

Summary:
Remove chronik utxo mgmt from ecash-coinselect. Focus on functionality of original coinselect library to support transaction building for app developers. Implement accumulative utxo selection algorithm.

Summary of changes from coinselect

• Use eCash dust threshold
• Convert input utxos value key from string (given by chronik) to number (used by TransactionBuilder)
• Remove integer and infinity checks from coinselect (assume input utxos from chronik are not corrupted)
• Replace var declarations with let
• Ignore chronik slp utxos
• Throw errors if inputs and outputs cannot be determined (vs behavior of coinselect, which returns {fee} instead of {inputs, outputs, fee} when this happens

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, emack

Differential Revision: https://reviews.bitcoinabc.org/D14526