[ecash-coinselect] Patch bug where returned outputs could be non-integer

Summary:
Library must return outputs in satoshis, which must be integer values. Non-integer fee rates can create decimals.

Round up when calculating the fee as you must pay sufficient fee, and the output should be correspondingly lower as the fee is implied.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14916