[appserver-io/appserver] Virtualize Filesystem

[wagnert]

In foresight to cluster functionality a filesystem virtualization is necessary.

Therefore all functions to that access FS has to be replaced with a integration of flysystem library and a VFS. This should make writing of PHPUnit tests simpler and enables appserver.io to support virtual filesystems.

• VFS Library MUST replace all native PHP filesystem functionality
• VFS Library MUST allow specify different virtualization type per directory
• VFS Library MUST provide high performance
• VFS Library MUST support Plug-In functionality
• Recommended Library is Flysystem