DiskCache: clean in smaller batches
DiskCache: clean in smaller batches

Cleaning in a single transaction would starve the readers
for a long long time.

Thib17/biggraphite,criteo/biggraphite,iksaif/biggraphite,Thib17/biggraphite,iksaif/biggraphite,criteo/biggraphite,iksaif/biggraphite,dpanth3r/biggraphite,iksaif/biggraphite,Thib17/biggraphite,Thib17/biggraphite,criteo/biggraphite,criteo/biggraphite