bug 2253: improve rebuild_zebra's handling of zebraqueue

bug 2253: improve rebuild_zebra's handling of zebraqueue

Prior to this patch, rebuild_zebra.pl -z was effectively
hanging on to a lock on the zebraqueue table, preventing
other scripts from inserting new entries into the table.
This had the effect of causing circulation operations
to time out.

Refactored by having rebuld_zebra.pl pull the active
queue into memory, then mark entries done by zebraqueue.id.
Consequently, rebuild_zebra.pl should no longer
block adding new entries into zebraqueue.

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