Add option to disable hashmap

[1.6] string: fix max_size() return value

string: change sso_capacity so that sizeof(string) is always 32 bytes

(which is minimal possible size and equal to sizeof(vector)) and
change way of sso and large string detection.

common: fix Coverity skips with proper variable

[1.6] Add missing include to v.hpp.

Needed by std::forward_as_tuple.

common: correction of 'equal to' and quotation marks

New members order makes it possible to use union and exploit vector
_size member to store some additional information (e.g. on MSB).

Merge back changes from Stable 1.6

common: sort arguments alphanumerically in Dockerfiles

test: add iterator tests for string

Reason: do not test undefined behaviour

string: separate declaration and definition

[1.6] cmake: require c++11 for cow check

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

[1.6] Bump pmdk to 1.6 in Docker images

C++ version was incorrectly computed by cmake to c++98 which resulted in compilation error on ubuntu 16.04

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