Radix tree improvements

radix_tree: refactor tagged_node_ptr to use self_relative_ptr

Previous type was copied from vmemcache and is too small.

std::atomic specialization for self_relative_pointer 

After changing radix_tree iterators API to return reference to the value
instead of a wrapper user should be able to check the capacity of an
inline string.

The storage cost of one additional field is not very high and it also
allows for checks inside assign() methods of inline_string.

and enable swap tests for radix_tree

Enable map access tests

Remove unused get_allocator code according to issue #827

Enable access tests ported from LIBCXX

Enable unclassified map tests

self_relative_ptr: fix assignment

Enable unclassified tests ported from LIBCXX

separate doc snippets

Ignore more libunwind errors

On most architectures, libunwind reuses getcontext and setcontext from
the C standard library.  memcheck report errors from these setcontext
calls.
This has been reproduced on ppc64le.

[1] https://github.com/libunwind/libunwind#libc-requirements

unify usage of get_pool()

Singlethreaded radix tree implementation

Also, add doc snippet (BytesView example) to radix_tree.

It reads a sequence of keys from stdin and prints radix in a DOT
format.

Based on: https://github.com/pmem/pmdk/blob/master/src/libpmemobj/critnib.h

It is a data structure that holds sequence of chars within the same
memory as the inline_string data structure itself. It requires
custom allocation logic and a runtime wrapper (accessor) which
is aware of the memory location and capacity.

Exclude self_relative_ptr from installation files

Implementing self-relative pointer