[alias-server] [p15] Only add alias records if alias is unique

Summary:
T2922

Depends on D13151

Only add alias records that are not already in the database

Test Plan:
sudo systemctl start mongod
In mongod, delete the ecashAliases database:

use ecashAliases
db.dropDatabase()

Run node index.js in web/alias-server
Note aliases are added to database
Run node index.js again
Note no aliases are added but all are processed

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13152