IMPORTANT: added -z option to rebuild_zebra.pl

IMPORTANT: added -z option to rebuild_zebra.pl

The -z option, when used in conjunction with -a and/or -b,
selects the records to reindex from the zebraqueue table.
Both record updates and record deletes are handled.

-z is cannot be used with -s or -r: the updated records
must always be freshly exported, and if zebraqueue
is to be processed, it's assumed that you don't want
to drop the Zebra index first.

This means that rebuild_zebra.pl -b -a -x can be
used as a cronjob to update the indexes periodically; it
is believed that this will offer much better indexing
performance on some setups as compared to zebraqueue_daemon.pl,
which uses Z39.50 extended services to send record updates
to Zebra.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>