ratelimiter: use a fake clock in tests and style cleanups

The existing test would occasionally flake out with:

	--- FAIL: TestRatelimiter (0.12s)
	    ratelimiter_test.go:99: Test failed for 127.0.0.1 , on: 7 ( not having refilled enough ) expected: false got: true
	FAIL
	FAIL    golang.zx2c4.com/wireguard/ratelimiter  0.171s

The fake clock also means the tests run much faster, so
testing this package with -count=1000 now takes < 100ms.

While here, several style cleanups. The most significant one
is unembeding the sync.Mutex fields in the rate limiter objects.
Embedded as they were, the lock methods were accessible
outside the ratelimiter package. As they aren't needed externally,
keep them internal to make them easier to reason about.

Passes `go test -race -count=10000 ./ratelimiter`

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

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