test: add iterator tests for string

Reason: do not test undefined behaviour

string: separate declaration and definition

common: update check-licence to the newest one from PMDK

test: add tests for string accessors

Move finding valgrind to top level CMakeLists.txt

to allow compiling examples with valgrind support.

Reason: we do not support custom allocators

test: add misc tests

string: extend pmem::string API to work with std::string

Persistent concurrent hash map data structure.

Implementation is based on concurrent_hash_map from Intel TBB library.

Test for persistent_concurrent_hash_map is built and executed only if
Intel TBB library found by CMake.

Update README for Intel TBB related tests.

Extend Docker images to support Intel TBB

Add abstraction for RW mutex

Add pmreorder test for concurrent_hash_map

Merge back changes from stable-1.6

Introduce options to enable array/vector/string

Merge back changes from stable-1.5

Adjust TRAVIS_COMMIT_RANGE variable to contain only commits from a PR.

to contain only commits from a PR.

Ref: travis-ci/travis-ci/issues/4596

Limit job configurations run on travis.

[1.5] cmake: check if dot tool is installed

fix doxygen invalid \ref command

which is required by doxygen to generate graphs

fix clang compilation error

[1.5] fix typo

clang 7.0.0 triggers -Wself-assign-overloaded in case of explicit self assignment
whilst we handle this in our implementation

All words in the documentation that correspond to a documented
class and contain at least one non-lower case character will
automatically be replaced by a link to the page containing the
documentation of the class.

Merge back changes from stable-1.5

Add changelog for 1.6