Cherry-pick of 5d671a8 onto stable5.

recreate an etag within the scanner if the cache contains an empty etag

Conflicts:
	lib/files/cache/cache.php
	tests/lib/files/cache/scanner.php
Emulate touch() for backends that don't support it

Backport of 9738fae3cf1ad18593d21eb62e138e00c01f5f36
Fixed testTouch unit test to work with stable5

Calling getFileInfo() would trigger checkUpdate() and would return the
real file mtime because the test value 500 was in the past.

This fix makes this test work with the stable5 behavior.

Removed "storage_mtime" references which don't exist in stable5.
Merge branch 'stable5' into fixing-4866-stable5
Merge branch 'fixing-4866-stable5' of github.com:owncloud/core into fixing-4866-stable5