new(lct): Lightweight Communication Tools

[JiakunYan]

This PR splits the original LCI library into two libraries: LCI and LCT (Lightweight Communication Tools).

I moved part of the non-communication-related functionality into the LCT library, with the hope that we can reuse the code in other libraries/benchmarks without rewriting them (just including lct.h). In addition, this makes it possible to benchmark individual parts of the LCI library (such as the completion queue).

Currently, the functionalities in the LCT library include:

• timing.
• string searching and manipulation.
• query thread id and number.
• logging.
• performance counters.
• different implementation of queues.

I choose to implement LCT in C++ since it is more convenient. LCT still has a C API.

Regarding performance overheads: we lost some opportunities for compiler optimizations across function calls (unless we do link-time optimizations), but since most of the functionalities here are not on the critical path (except thread id and queues), I think it should be fine. I did not notice any visible performance loss with HPX microbenchmarks and Octo-Tiger.

Eventually, we can gradually move all the code from LCI to LCT and make LCI a C++ library (with a C API).

Other changes:

• I changed the environmental variable LCM_LOG_LEVEL to LCI_LOG_LEVEL.
• I changed the default completion implementation from CAS based to FAA based. It will give a much better performance to multithreaded pop. Users can use the environmental LCI_CQ_TYPE to switch to another implementation or the cmake option LCI_USE_INLINE_CQ to switch back to the C implementation.
• I refactored the implementation of the performance counters. It is much more generic and can serve also as a timer.

[JiakunYan]

@omor1 Let me know if you have any comments before I merge this.

[omor1]

@omor1 Let me know if you have any comments before I merge this.

I'm still out at ICPP, not sure when I'll have time to check this.