Introduce volatile_state support class

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).