Update readme

Use COW support from pmdk-1.6

Merge back changes from stable-1.7

Throw transaction_scope_error in case vector/string ctor was called o…

[1.7] Fix compilation of concurrent hash map test

[1.7] Fix void_t declaration.

Fixes: https://github.com/pmem/libpmemobj-cpp/issues/366

Update test to new hashmap API introduced in: c67264ae

Drop libdir from pkgconfig.

Extend messages in exceptions

Until CWG 1558 (a C++14 defect), unused parameters in alias templates were
not guaranteed to ensure SFINAE and could be ignored, this implementation
of void_t solves this issue. See: https://en.cppreference.com/w/cpp/types/void_t

to message argument. Use those exception in every place where pmemobj error
is handled.

Speed up some DEVELOPER_MODE checks.

cmake: fix tbb detection for distros without tbb cmake integration

There's no binary library to link against.

string: fix memcheck error of uninitialized size field

Merge back changes from stable 1.6

docker: bump OS' versions

Heterogeneous lookup in concurrent_hash_map

include make_persistent_array.hpp

tests: fix build

fix missing include header file

docker: bump valgrind version

Extended concurrent_hash_map to support heterogeneous find/count/erase
operations. Our implementation is based on the idea described in ISO C++
paper P0919R3 for std::unordered_map
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0919r3.html).