filters: add API for better interaction with file objs
update other filters
Check if volume size fits in cluster capacity (bb#668)

Fix tests to honour checking if volume fits in cluster capacity, used
volume size was exceeding cluster capacity.

Add fcgi test for checking if volume is not too big during its creation.
Introduce volume size enforcement (bb#669)

Add test for volume size enforcement.
Do not upload recursively when we have 413 status
Report current volume usage