[alias-server] Reject alias registrations that are case variants of previously registered valid aliases

Summary:
T2967

Per latest updates to alias spec. Do not allow mixed case variants of aliases. For example, if a user registers "MyNewAlias" at blockheight 800000, no one else can later register "mynewalias" or "MYNEWALIAS"

Test Plan:
Review code change and new unit test
npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13227