* Make implementation header only
* Opaque implementation in namespace
* Use std::atomic instead of custom implementation
* Modify ringbuf test to work with c++ changes

Importi multi-producer single-consumer (MPSC) ring buffer from https://github.com/rmind/ringbuf

Implement defer free for radix_tree

and enable overwrite tests (since runtime_initialize_mt is
available).

which enables single-writer multi-reader concurrency. After calling
runtime_initialize_mt() each assignment (done through assing_val or
insert_or_assign) does not modify the value in-place but rather
allocates new node. Also, erase does not free nodes/leaves immediately
but adds them to a garbage list which can be processed using
garbage_collect.

gha: temporarily disable codecov

gha: temporarily disable windows builds

Update dockers

and clean up images a little.

Various fixes

radix_tree: use std::atomic<tagged_pointer>

add readme to benchmarks and fix dividing by zero in radix bench

tests: fix sign-conversion warning in helpers_test

add benchmark which compares radix to std::map

Make syncthreads() in parallel_xexec multi-use

Previously syncthreads could only be used once and this was not
documented. This patch allows for having multiple synchronization
points in a function. This might useful for testing hazard pointers.

Add example for using pool as a class member

CMake fixes

version comparison should be used, instead of string comparison.
Simplify the function, by the way.

- extend print messages,
- print warnings once per build, where possible,
- add more comments,
- fix minor todo's,
- add cache string for TBB_DIR.

Clean docker images