Throwing exceptions from code sometime confuses ASAN, as it cannot keep track of stack frames. This change therefore adds a macro to skip instrumentation around the `Throw` function.
ASAN wasn't able to keep track of `boost::coroutine` context switches, and would lead to many false positives being detected. By switching to `boost::coroutine2` and `ucontext`, ASAN is able to know about the context switches advertised by the `boost::fiber` class, which in turn leads to more cleaner ASAN analysis.
