adding size() to the file cache
returning the number of stored bytes in store() and adding cleanup() method
adding detection of aborted uploads

Conflicts:
	lib/connector/sabre/directory.php
minimize diff for easier review
Merge branch 'stable5' into backport-4867-stable5
fixes #5367

Conflicts:
	lib/connector/sabre/file.php
fixes #5367

Conflicts:
	lib/connector/sabre/file.php
Merge pull request #5327 from owncloud/backport-4867-stable5

[stable5] Adding detection of aborted uploads for chunked uploads