autorelay: fix busy loop bug and flaky tests in relay finder

[MarcoPolo]

Also adds our own custom mock clock that fits a better interface for this module.

The new mock clock implements the following interface:

// InstantTimer is a timer that triggers at some instant rather than some duration
type InstantTimer interface {
	Reset(d time.Time) bool
	Stop() bool
	Ch() <-chan time.Time
}

// ClockWithInstantTimer is a clock that can create timers that trigger at some
// instant rather than some duration
type ClockWithInstantTimer interface {
	Now() time.Time
	Since(t time.Time) time.Duration
	InstantTimer(when time.Time) InstantTimer
}

This lets us avoid the issue of #2187 altogether.

The new mock clock is about 100 LOC and should behave logically close to a real clock (time shouldn't jump from 0 to 1000 without work being done in between).

Closes #2187. Closes #2192. Fixes #2189. Closes #2183

I chose to incorporate those PRs into this one to avoid the merge conflicts that would ensue.

[marten-seemann]

Oh nice, you already extracted it to a separate package.