[Cashtab] Make tx fee in sats/byte a param
Summary: Add feeInSatsPerByte as a param in the sendBch function. This allows unit tests to use a constant.
Test Plan: npm test
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D9088