Update io_service::strand to satisfy the Executor requirements.
Make asio::spawn() executor-aware.
Delay allocator rebind to keep symbol names shorter.
Add Executor type requirements.
Fix up small block recycling.
Fix executor_wrapper's call operator for older C++ compilers.
Fix workaround for clang/libc++.
Merge remote-tracking branch 'upstream/master'
Add missing reference to boost thread - coroutine in boost 1.56 now needs it