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

because PMDK is installed from OS'es package repository.

do all updates and packages installs in one RUN;
and create user and password in one step as well.

and images' cleanups.

update docs

so it will be available in the generated docs.

Add extra Windows CI job

examples: extend radix_tree_complex_value example

tests: add double-create test for pool

to show that creating city_info on dram is not possible and show
example with a structure wrapped in pmem::obj::p<>

it should fail with pool_invalid_argument, since it returns EEXIST.

Fix a bunch of typos.