writableTail should not be const

Summary: A `const IOBufQueue` should not leak mutable references to its innards. This can be a footgun. For example, `const` conventionally means "thread-safe". Writing through `writableTail` on a `const IOBuf*` that's shared across threads is anything but safe.

Reviewed By: ot

Differential Revision: D58380170

fbshipit-source-id: 4909624e650b3a8c2185167a68fd39b8b1eaa1a9

folly/io/IOBufQueue.h

@@ -436,7 +436,7 @@ class IOBufQueue {
    * @brief Get a pointer to the writable tail section.
    * @methodset Access
    */
-  void* writableTail() const {
+  void* writableTail() {
     dcheckCacheIntegrity();
     return cachePtr_->cachedRange.first;
   }