[contrib/mmap] implements a theoretical dynamic growing mmap

[contrib/mmap] implements a theoretical dynamic growing mmap
based on Michael Vogts patch in #195018 this commit implements the use
of mmap (as preferred) instead of a static char array. In theory this
made it possible to grow the mmap as needed, but as it is currently
impossible to move the mmap around in the memory the grow is likely
to fail but it improve the memory usage a bit, so it is not totally
useless for now - and maybe we can adjust the pointers in the future...