coro::thread_pool uses lockless mpmc queue

std::deque with std::mutex and std::condition_variable are easy to use
but atrocious for performance. Using a relatively drop in replacement
moodycamel::BlockingConcurrentQueue to speed up the scheduling of tasks
onto a thread pool.

Noted items:
* a coroutine in a while(cond) { co_await tp->yield(); } loop where the
  thread_pool only has 1 executor thread causes an infinite loop, I've worked
  around this by using `queue.wait_dequeue_bulk_timed()` to guarantee
  multiple tasks get executed per executor loop, this is not done with
  multiple executor threads since we want to distribute the work more
  evently.

Closes #404

Author: jbaldwin

CMakeLists.txt

@@ -114,6 +114,10 @@ set(LIBCORO_SOURCE_FILES
     include/coro/concepts/promise.hpp
     include/coro/concepts/range_of.hpp
 
+    include/coro/detail/vendor/cameron314/concurrentqueue/concurrentqueue.h
+    include/coro/detail/vendor/cameron314/concurrentqueue/lightweightsemaphore.h
+    include/coro/detail/vendor/cameron314/concurrentqueue/blockingconcurrentqueue.h
+
     include/coro/detail/awaiter_list.hpp
     include/coro/detail/task_self_deleting.hpp src/detail/task_self_deleting.cpp
     include/coro/detail/void_value.hpp

include/coro/detail/vendor/cameron314/concurrentqueue/LICENSE.md

@@ -0,0 +1,62 @@
+This license file applies to everything in this repository except that which
+is explicitly annotated as being written by other authors, i.e. the Boost
+queue (included in the benchmarks for comparison), Intel's TBB library (ditto),
+dlib::pipe (ditto),
+the CDSChecker tool (used for verification), the Relacy model checker (ditto),
+and Jeff Preshing's semaphore implementation (used in the blocking queue) which
+has a zlib license (embedded in lightweightsempahore.h).
+
+---
+
+Simplified BSD License:
+
+Copyright (c) 2013-2016, Cameron Desrochers.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice, this list of
+conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice, this list of
+conditions and the following disclaimer in the documentation and/or other materials
+provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
+OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
+TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+---
+
+I have also chosen to dual-license under the Boost Software License as an alternative to
+the Simplified BSD license above:
+
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
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+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.