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.

tests: run concurrent map gdb test only on gcc

Update cmake_build_type handling and switch default to RelWithDebInfo

it should now produce warnings, when it shouldn't be run. This way
we make sure our CMake warns users properly.

This test doesn't work properly with Clang compiler (rbreak seems to not work),
Since it works well with gcc it's better to run it only on gcc and print a warning
otherwise, so user doesn't run it accidentally.

Fixes #968

add pool_invalid_argument exception