Make AsyncIO::capacity_ const

Summary: It is set once, and is never changed afterwards.

Test Plan: compiled, unit tests

Reviewed By: tudorb@fb.com

FB internal diff: D706125

folly/experimental/io/AsyncIO.h

@@ -167,7 +167,7 @@ class AsyncIO : private boost::noncopyable {
 
   io_context_t ctx_;
   size_t pending_;
-  size_t capacity_;
+  const size_t capacity_;
   int pollFd_;
   std::vector<Op*> completed_;
 };