Allowing persistent containers to be stored in volatile memory.

[igchor]

Currently, persistent vector, array and string are not allowed on volatile memory (including stack). This makes it impossible to pass/return those containers by value. This limitation was introduced to decrease possibility of a persistent memory leak. Persistent containers always allocate from pmem so if a crash would occur before the container is moved to persistent memory we would have no way to access memory which was managed by the container.