Allocator class stub

[treywood-qz]

We recently had a candidate come into the backend interview set up to work in Ruby on Rails. That's fine and dandy except the candidate leveraged a Postgres database and the Rails ORM to do all of the heavy lifting. I think having such a bare example in this repo can set the wrong expectation coming into the interview, so I've added an Allocator class stub here that could hopefully clarify the expectation that we will ignore the web framework for a portion of the interview.

I wanted to also add something like

constructor() {
  this.servers = [];
}

to emphasize work to be done in-memory, but felt like that might be a bit too suggestive - thoughts?