tests: enable map.ops tests which require generic std::less

add CONTRIBUTING.md file

Intead of std::less<void> which is available from C++14 use
transaprent less which has the same implementation.

Leave just one test with std::less<> as sanity check.

Enablement of generic erase test

- Add helper header for map-like structures
- Enable erase_key test

LIBCXX map tests: OPS

Add ops tests taken from libcxx master branch

get rid of old WA for vcpkg and use latest release instead

add document describing release steps

Add headers taken from libcxx

tests: fix volatile_state test

pmemobj_close is not thread safe and cannot be called concurrently with
pmemobj_direct.

disable drd test

disable concurrent_map_insert_reopen drd tests due to intermittent
failres (missing annotation, see issue https://github.com/pmem/libpmemobj-cpp/issues/770)

Fix sporadic DRD issue in concurrent skip list

concurrent_map: fix windows compilation

Fix Doxygen warnings in the concurrent_skip_list_impl.hpp

concurrent_map fixes

otherwise thread 11 and 12 might not start when we try to switch
to them in gdb script.

_size is a volatile variable and does not have to be flushed.
pmemcheck complained about missing flush in pmemkv tests.

Fix misleading warning message

add issue templates for GitHub interface

concurrent_map: fix windows compilation

On older MSVC versions sizeof(member) is not allowed in definition
of non-static members. Fix this by adding decltype().

Concurrent map data structure

Make pmem::allocator::allocate() throw exceptions in case of bad
allocation.
Make pmem::allocator::construct() throw exception if it is called
outside of transaction.