Specifically:

LLCoros::Mutex means boost::fibers::mutex
LLCoros::LockType means std::unique_lock<boost::fibers::mutex>
LLCoros::ConditionVariable means boost::fibers::condition_variable
LLCoros::cv_status means boost::fibers::cv_status

So as not to drag in all of boost::fibers::mutex.hpp or condition_variable.hpp
for each consumer of llcoros.h, instead #define LLCOROS_MUTEX_HEADER and
LLCOROS_CONDVAR_HEADER. Those who need them can #include the relevant macro.

Update llcond.h and llthreadsafequeue.h accordingly.

Also introduce value_type typedef.

LLCond encapsulates the usage patterns required to properly use
condition_variable. We also provide LLScalarCond, LLBoolCond and LLOneShotCond.