fix CMake issue

cmake 3.3 (which we're using) doesn't have function GREATER_EQUAL

enumerable_thread_specific: remove unnecessary include

Ref: https://github.com/pmem/pmemkv/issues/628

Remove script for updating vcpg's PMDK version

Vcpkg already has PMDK 1.8

Merge stable-1.8 into master

Update docs in defragmentation class

Update PMDK dependency to 1.8

Merge stable-1.7 into stable-1.8

Merge stable-1.6 into stable-1.7

Create separate Docker images for stable-1.8 branch

Create separate Docker images for stable-1.7 branch

Create separate Docker images for stable-1.6 branch

update README files

Fix basic_string.access tests

Update READme.md and include/libpmemobj++/README.md with an
information about containers. Fix compiler requirements.

fix: adjust UT_ASSERT to gcc 7.5.0 from openSUSE Leap

Create separate Docker images for v1.9 branch

Changes made in test were not persistent and failed on pmemcheck.

It fixes the following compilation error:

In file included from /libpmemobj-cpp/tests/concurrent_hash_map_tx/concurrent_hash_map_tx.cpp:38:0:
/libpmemobj-cpp/tests/concurrent_hash_map_tx/concurrent_hash_map_tx.cpp: In function 'void test_tx_singlethread(pmem::obj::pool<root>&)':
/libpmemobj-cpp/tests/concurrent_hash_map_tx/concurrent_hash_map_tx.cpp:359:27: error: conversion to 'pmem::obj::concurrent_hash_map<pmem::obj::p<int>, pmem::obj::p<int> >::size_type {aka long unsigned int}' from 'int' may change the sign of the result [-Werror=sign-conversion]
  UT_ASSERTeq(map->size(), static_cast<size_t>(number_of_inserts));
/libpmemobj-cpp/tests/common/unittest.hpp:151:21: note: in definition of macro 'UT_ASSERTeq'
  ((void)(((lhs) == (rhs)) ||                                            \
                     ^~~

Update PMDK to test the last defrag fixes

Remove CMake checks for pmemvlt (we require libpmemobj >= 1.7 anyway)

Klockwork and coverity fixes

Merge stable 1.8 into master

compatibility tests: test with 1.8.1 instead of 1.8