Add support for stackless HTTS.
HTTS v2
Move htts_v2 to local
Remove outdated HTTS scripts, rename general-purpose HTTS to timed task spawn

Conflicts:
	tests/performance/hpx_homogeneous_timed_task_spawn.cpp
	tests/performance/local/hpx_homogeneous_timed_task_spawn.cpp
	tests/performance/local/htts/.gitignore
	tests/performance/local/htts/counters.cfg
	tests/performance/local/htts/plot_htts_strong.gpi
	tests/performance/local/htts/plot_htts_strong.sh
	tests/performance/local/htts/plot_htts_weak.gpi
	tests/performance/local/htts/plot_htts_weak.sh
	tests/performance/local/htts/postprocess_htts.py
	tests/performance/local/htts/run_htts.sh
	tests/performance/timed_task_spawn.cpp
Add qthreads implementation, minor cleanup.
Move htts2_qthreads to local/
Fix headers, compilation errors
Add htts_v2 to build

Conflicts:
	tests/performance/CMakeLists.txt