[Cashtab] Calculate max send amount in appropriate way using updated ecash-coinselect

Summary:
We are using a legacy function to calculate the fee here that is underestimating the actual max send amount.

Using an updated helper function from ecash-coinselect, calculate the max send amount appropriately (including consideration for a cashtab msg, if present).

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15553