Epic: File API improvements to handle clustering environments

This has been talked about for many years but appears not to have an issue filed for it.

Moodle Files currently go into dataroot directory on the server filesystem.

This can cause problems when Moodle is used in server cluster environments where there may not even be local filesystems, when synchronisation is required across potentially many web servers.

This issue is the starting point for solving this problem.

1) Collect solutions/ideas/hacks from the community and discuss.
2) Define a specification for core implementation in Moodle
3) Split into subtasks under this issue.
4) Complete the implementation.