[alias-server] Return pending registrations for a specified alias

Summary: Would be useful for a registering user to know if there are pending registration txs already on the network for an alias he is attempting to register

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D14683