Fix minor comments issues

volatile_state: add missing include

test: fix one type in volatile_state test

Check license only in modified files

It fixes the following compilation error:

In file included from /libpmemobj-cpp/tests/volatile_state/\
volatile_state.cpp:33:0:
/libpmemobj-cpp/tests/volatile_state/volatile_state.cpp:\
In function 'void test_vector_of_elements(pmem::obj::pool<root>&)':
/libpmemobj-cpp/tests/volatile_state/volatile_state.cpp:348:30:\
error: conversion to 'size_t {aka long unsigned int}' \
from 'std::__atomic_base<int>::__int_type {aka int}' \
may change the sign of the result [-Werror=sign-conversion]
  UT_ASSERT(v2_initialized == NUM_ELEMENTS);
                              ^

Make persistent_ptr_base non-template and move to main namespace

with two more cases, explicitly using converting assignment and static_cast.

because it was never used, and probably was erroneous (not re-calculating offset).

Since we want to widely use non-template version of base class it's
better to have it in main (pmem::obj) namespace.
Thanks to that it can be used as a type e.g. in vector to
store pointers of various (template) persistent_ptr elements.

persistent_ptr_base is changed to enable usage e.g. as a generic
parameter for a function requiring any persistent_ptr.
Non-template version of the base class is perfect for that.
Rationale for this change is a defragmentation feature, which will operate on
various persistent_ptr's.

Introduce volatile_state support class

And explicitly set CXX_STANDARD for each build (also set name of the
build functions in run-build.sh accordingly).

there is a regression on devel-1.8 which causes compilation error
when we do 'sudo make -j$(nproc) install'. This is a workaround.

It allows persistent object to use volatile data. It is
implemented as global volatile hash_map which maps PMEMoids
to pointers to volatile data. The hash_map is protected by
read write lock (available from C++14 hence this is the
requirement for the entire feature).

Volatile state stored in hash_map is cleared on pool close
(only the state which is conncted to pmem objects from the
closed pool).

Ignore the patch part of a program's version

this is neccessary for volatile_state implementation.

to pool and transaction classes description.

It's now possible to register callback for transaction. This implementation
uses pmemobj C tx callback internally but the user is able to
register multiple callbacks for each transaction stage.

Do not run anything on appveyor

The patch part of a program's version almost never matters.

This change is needed to accept clang-format v9.0.1 when v9.0 is required.

Windows builds are now being run on github actions.

Add running tests on Github Actions for Windows

Merge changes from stable-1.8

Not passing required flags caused error on windows. Also including
libpmemobj.h cause inclusion of type macros which would case problems
with offsetoff deifnition on newer VS.

Concurrent hash map does not depend on v<>.

this allows vcpkg integrate install to find the libraries. This allows
to run cmake without the need for specifying CMAKE_PREFIX_PATH