Use an enum to represent faucet queue state changes

[jbearer]

Currently, there are 3 state changes persisted on disk:

• Some(0) -- a new request was created
• Some(n > 0) -- a request has received its nth grant
• None -- a request was completed

We can make the code more clear using an enum like

enum QueueOp {
  New,
  Grant(n),
  Complete,
}