[alias-server] Accept overpaid tx fees

Summary:
T2961

Recognize overpaid registration fees as valid. Bugfix discovered in authoring this diff: txs without OP_RETURN should return false from parseAliasTx

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13206