Enable boost handler tracking via compile-time flag

[halfalicious]

Boost ASIO includes a handler tracking feature which one can enable by defining BOOST_ASIO_ENABLE_HANDLER_TRACKING (see https://www.boost.org/doc/libs/1_47_0/doc/html/boost_asio/overview/core/handler_tracking.html). This functionality is very useful for debugging handler issues (e.g. handler-related use-after-frees) so it would be nice to be able to enable it at compile time via a CMake option.