Migrate to CLA Assistant; fix flaky test

CLA Hub silently disappeared, along with the signatures it gathered.
Thankfully the Apache License covers all cases, such as contributions,
so this was not required. We offer a CLA in case if alleves concerns
by a corporate legal review. The same CLA terms apply: a direct copy
of Apache's with the names updated.

A test appears flaky on the CI due to weak/soft values being collected
during its execution. The test now better ensures that the entries are
not collected, hopefully stabilizing this test case.

CONTRIBUTING.md

@@ -1,4 +1,4 @@
-To get started, [sign the Contributor License Agreement](https://www.clahub.com/agreements/ben-manes/caffeine).
+To get started, [sign the Contributor License Agreement](https://cla-assistant.io/ben-manes/caffeine).
 
 Then head over to [How to Contribute](https://github.com/ben-manes/caffeine/wiki/Contribute)
 for details on the build system.

caffeine/src/test/java/com/github/benmanes/caffeine/cache/ExpirationTest.java

@@ -185,16 +185,19 @@ public void schedule_delay(Cache<Integer, Duration> cache, CacheContext context)
     doAnswer(onRemoval).when(context.removalListener()).onRemoval(any(), any(), any());
     when(context.scheduler().schedule(any(), task.capture(), delay.capture(), any()))
         .thenReturn(Futures.immediateFuture(null));
+    Map<Integer, Duration> original = new HashMap<>();
 
     Integer key1 = 1;
     Duration value1 = Duration.ofNanos(context.ticker().read());
+    original.put(key1, value1);
     cache.put(key1, value1);
 
     Duration insertDelay = Duration.ofMillis(10);
     context.ticker().advance(insertDelay);
 
     Integer key2 = 2;
     Duration value2 = Duration.ofNanos(context.ticker().read());
+    original.put(key2, value2);
     cache.put(key2, value2);
 
     Duration expireKey1 = Duration.ofNanos(1 + delay.getValue()).minus(insertDelay);
@@ -207,7 +210,7 @@ public void schedule_delay(Cache<Integer, Duration> cache, CacheContext context)
 
     Duration maxExpirationPeriod = Duration.ofNanos(
         context.expiryTime().timeNanos() + Pacer.TOLERANCE);
-    assertThat(actualExpirationPeriods, is(aMapWithSize(2)));
+    assertThat(actualExpirationPeriods, is(aMapWithSize(original.size())));
     assertThat(actualExpirationPeriods.get(key1), is(lessThanOrEqualTo(maxExpirationPeriod)));
     assertThat(actualExpirationPeriods.get(key2), is(lessThanOrEqualTo(maxExpirationPeriod)));
   }