Boost fiber

[pmconrad]

Proof-of-concept for switching fc::thread to boost::fibers . Replaces fc::do_parallel with boost::fibers constructs. Related tests are working.
Still much to do.
Requires Boost-1.62+.
boost::fibers contains a memory leak in algorithm_with_properties that still needs to be worked around.

[abitmore]

Boost.Fiber depends upon Boost.Context. Boost version 1.61.0 or greater is required.

Ubuntu 16 users would need to build boost by themselves.

[pmconrad]

Yes. I think by the time this gets into production Ubuntu-20.04 will be out. We can consider bumping requirements to Boost-1.62 and C++17 then.

[pmconrad]

Got rid of the memory leak. All tests working locally.

There are some key differences to the old fc::thread implementation:

• A thread that wants to receive async task assignments from other threads must call fc::initialize_fibers() first.
• A task that is being pushed into another thread will only be pushed when the current fiber yields.
• A thread that still has fibers in its queue will not terminate, even if the "main" fiber exits. This is particularly nasty for background fibers started from the main thread - they will prevent the program from exiting, even if the main fiber has left main().
• Unlike fc::task, there is no support for cancelling a fiber. A means to cancel a task that is scheduled in the background must be implemented separately. Due to this, fc::schedule should not be used with a time in the far future.
• boost::fiber synchronization mechanisms (mutexes and condition variables) do not work in destructors of objects with static lifetime.