add copy assignment operator to BasicDynamicTokenBucket

Summary: `BasicDynamicTokenBucket` declares a copy constructor, but not copy assignment operator.

Differential Revision: D30258099

fbshipit-source-id: c993177e11023d13d01884e5060269c4d5813088

folly/TokenBucket.h

@@ -275,13 +275,15 @@ class BasicDynamicTokenBucket {
       : bucket_(zeroTime) {}
 
   /**
-   * Copy constructor.
+   * Copy constructor and copy assignment operator.
    *
    * Thread-safe. (Copy constructors of derived classes may not be thread-safe
    * however.)
    */
-  BasicDynamicTokenBucket(const BasicDynamicTokenBucket& other) noexcept
-      : bucket_(other.bucket_) {}
+  BasicDynamicTokenBucket(const BasicDynamicTokenBucket& other) noexcept =
+      default;
+  BasicDynamicTokenBucket& operator=(
+      const BasicDynamicTokenBucket& other) noexcept = default;
 
   /**
    * Re-initialize token bucket.