kohabug 2048 - improve zebraqueue_daemon performance

kohabug 2048 - improve zebraqueue_daemon performance

[1] Increase sleep interval between checks of zebraqueue
    from 0.01 seconds to 0.50.
[2] Batch up commits of changes to the zebraqueue table
[3] If the same record appears multiple times in the queue,
    handle only once.
[4] Properly postpone failures to process record deletes to
    avoid spinning.
[5] Correct how queue entries are marked done - avoid skipping
    an authority record update, e.g., if it has the same
    ID number as a bib that was updated.
[6] Added a FIXME about a possible later enhancement to
    batch up updates so that Zebra isn't told to commit
    after each record.

No documentation changes.

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