Update docs in defragmentation class

Update PMDK dependency to 1.8

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

And explicitly snapshot my_mask and my_size in clear() and swap().
Those are the only 2 function in which we want size and mask to
be changed transactionally (because they can be called inside
outer transaction).

The exception can only be thrown in case of a bug, so assert this
condition.

structs with manual memory management should provide copy ctor.
Otherwise default generated one will perform a shallow copy and
there will be double free in dtor.

detail::pair: make piecewise ctor not constexpr

It should not be marked as constexpr because it does not
initialize all members directly (but through another ctor).

std::pair piecewise ctor is constexpr only in c++20.

Run find_gdb() in tests/CMakeLists.txt

It fixes regression introduced by bc3438ae.

Disable false-postivie helgrind tests

Disable the concurrent_hash_map_rehash_mock tests
due to intermittent failures (most probably false-positive
reported by helgrind).

Ref: 53c8568c
Ref: #469

Use https:// where appropriate.

concurrent_hash_map fixes