Improve vhange directory

created a static function that recieves the cdPath and destination.
now the User::moveFolder is just inline to the new static (which is private)
the User::getRealFile() is now const and much better (no need to bakcup the previous, work, restore) - now we just copy once to local and work on it.

tested - works great.
Convert File System functions to multithreading - P1

part 1
the getFileSize() should not be multithreaded because it is very fast!
all converted functions were tested
Merge branch 'master' of github.com:arthurzam/aftp-server

Conflicts:
	User.cpp