billing plots: use 'transferred' rather than 'size' for bytes

This bug has gone
undiscovered. The plotting for dCache
bytes read and written has been accessing aggregate table
values for fullsize rather than transfersize. While this
is OK for writes, reads will not be correctly represented
(we found this on our CMS Tier1 dcache, where
it was off by a factor of 10).

RELEASE NOTES:

    Fixes a bug in the billing plots causing incorrect values for dCache bytes read.

Target: 2.7
Require-book: no
Require-notes: yes
Acked-by: Dmitry
Patch: https://rb.dcache.org/r/7290
Committed: master@7680a0156a1af5b5479af7b4472c2385117cc711
Merge pull request #829 from alrossi/fix/2.7/bytes-read-plot

billing plots: use 'transferred' rather than 'size' for bytes