This allowed to avoid improper static_cast to check downcasting

Metadata class needs to be used by both the client and the daemon

The metadata class needs to represent metadata of a file,
thus it shouldn't contain the path itself. In fact we actually didn't
use it at all.

This will make further refactoring easier and it save some memory copies.

when unlinkat was called with a link we were removing the target and not
the link itself

In the kernel VFS some file descriptors under /proc/<PID>/fd/ could be
links to a pipe, this is a special case in which the `readlink` function
would not work. So when we detect this case we forward to kernel

when resolving path we don't want always
to resolve also the last components if it is a link. Cause in some cases
we want exactly the path of that link and not the one it points to.

lstat and readlink are some example of functions that don't want to
resolve the last link.

 - avoid error messages on mercury cleanup
 - fix spaces and indentation