LP#1717350: fix chunking messages sent from Perl services

LP#1717350: fix chunking messages sent from Perl services

The chunk size adjustments introduced by the patches for bug 1709710
could result in calculating non-integral chunk sizes for Perl services.
This in turn led to fractional offsets and lengths being passed to
Perl's substr() function; rounding in turn can lead to characters
in the response getting silently dropped, which in turn broke
parsing the reconstituted response.

To test
-------
The problem can be reproduced by invoking

  open-ils.storage open-ils.storage.actor.org_unit.descendants.atomic 1, 0

in Evergreen databases with org unit trees large enough that the
response gets chunked.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>