Binary search for users

[michaelKurowski]

Is your feature request related to a problem? Please describe.
LokIM should optimize searching by username, as it will be the most common used query.

Describe the solution you'd like
We should keep users in alphabetical order by username, and perform a binary search while querying for user with certain username.

Describe alternatives you've considered
N/A

User story
N/A

Additional context
Study materials:
Optimizing query performance in MongoDB