Bug 10955 - Add ability to skip deletions in zebraqueue

It seems that record deletions can cause extreme slowdowns for Koha
installations with extremely large numbers of records. It would be
helpful to be able to skip record deletions when processing the
zebraqueue with rebuild_zebra.pl so the deletions can be processed with
a lower frequency.

Test Plan:
1) Disable any zebra indexing cronjobs you may have
2) Delete a record
3) Note the operation recordDelete in the zebraqueue table having done = 0
4) Run misc/migration_tools/rebuild_zebra.pl -b -z --skip-deletes
5) Note the delete still has done = 0
6) Run misc/migration_tools/rebuild_zebra.pl -b -z
7) Note the delete now has done = 1

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script.
Also tested for authorities, no problems found.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>

RM note: this is at best a work-around, and I will emphasize that
--skip-deletes should be used only when absolutely necessary.

I hope that --skip-deletes can go away at some point soon, but
that may depend on changes to Zebra.
Bug 10955: (follow-up) improve usage information

This patch improves rebuild_zebra.pl's usage help
by explaining when --skip-deletes should be considered
and noting that it should be used in conjunction with
a cronjob to process deletions after hours.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>