No need to keep file handle open after memory mapping

[hendrikmuhs]

It is safe to destruct a file_mapping after mapped_region:

~file_mapping();

Destroys the file mapping. All mapped regions created from this are still valid. Does not throw

from: https://www.boost.org/doc/libs/1_46_1/doc/html/boost/interprocess/file_mapping.html

Therefore we can remove this member. It is not used outside of the constructor. There we can use a variable instead