Small state machine to handle potential bursts of validations caused by rapidly flipping states of the underlying connection. If a validation is requested while one is being run, we schedule a new validation to start when the current one completes. Extra requests are collapsed into this single scheduling.

                 request                       request
┌───────────┐   validation   ┌───────────┐    validation    ┌───────────┐
│   IDLE    ├────────────────►VALIDATING ├──────────────────►VALIDATION ├────┐
├───────────┤                ├───────────┤                  │  PENDING  │    │ request
│           │                │ on entry/ │                  ├───────────┤    │validation
│           │                │   start   │                  │           │    │
│           ◄────────────────┤validation ◄──────────────────┤           ◄────┘
└───────────┘   validation   └───────────┘    validation    └───────────┘
                 complete                      complete