Do not allow warmupPeriod set as zero

[liufuyang]

In order to solve issues like #2730
We encountered this "crazy" issue in production that when setting warmupPeriod=0, the ratelimiter changed the rate from 1 per second to millions (if not billions) per second.

Or simply see this issue by running following code as a test:

  @Test
  public void t() {
    RateLimiter rateLimiter = RateLimiter.create(1, 0, TimeUnit.SECONDS);

    while (true) {
      rateLimiter.acquire();
      System.out.println("Done");
    }
  }

When setting warmupPeriod=0 will trigger some downstream slope calculation with some value divide 0: https://github.com/google/guava/blob/master/guava/src/com/google/common/util/concurrent/SmoothRateLimiter.java#L229

[liufuyang]

@cpovirk Perhaps you can give a review of this?

[liufuyang]

@cgdecker Perhaps you can take a look as well? :)

[liufuyang]

@ronshapiro @kluever Pinging you two here as I saw you changed this ratelimiter recently 😄