hold targeter: add option to run parallel targeter processes

hold targeter: add option to run parallel targeter processes

Permit the hold targeter to divvy up the work and run more than one process
at a time by setting the opensrf.xml setting hold_targeter/parallel to a
value greater than one.  Doing so can significantly reduce the
time it takes to (re)target a large number of hold requests, although
only up to a point (in other words, if increasing the number of parallel
targeters beyond one, it is recommended to do so slowly.)

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
git-svn-id: svn://svn.open-ils.org/ILS/trunk@18815 dcc99617-32d9-48b4-a31d-7c20da2025e4