TokenBucketStorage primitive extracted from BasicDynamicTokenBucket

Summary:
Extract the underlying primitive that powers BasicDynamicTokenBucket, as TokenBucketStroage. This abstraction speaks to "tokens" (instead of time) and allows for arbitrary borrowing.  It also handles thread safety.

This allows other bucket token based libraries to be created (eg. a Cooperative Scheduler) using the same primitive.

API compatibility has been maintained with the existing classes.

Reviewed By: yfeldblum, rohithmenon

Differential Revision: D29522354

fbshipit-source-id: 010bd9caf3f9c3726621e87d23e5c564773424b7

folly/test/TokenBucketTest.cpp

@@ -35,6 +35,20 @@ TEST(TokenBucket, ReverseTime) {
   EXPECT_EQ(tokensBefore, tokenBucket.available());
 }
 
+TEST(TokenBucketTest, CtorAssign) {
+  BasicDynamicTokenBucket bucketA(100.0);
+  EXPECT_EQ(0, bucketA.available(10, 10, 90));
+
+  BasicDynamicTokenBucket bucketB(bucketA);
+  EXPECT_EQ(0, bucketB.available(10, 10, 90));
+
+  bucketA.reset(0.0);
+  EXPECT_EQ(10, bucketA.available(10, 10, 90));
+
+  bucketB = bucketA;
+  EXPECT_EQ(10, bucketB.available(10, 10, 90));
+}
+
 TEST_P(TokenBucketTest, sanity) {
   std::pair<double, double> params = GetParam();
   double rate = params.first;