Fixed queuing strategy in MemArchiveQueue::sharedAddToArchiveQueue().

We now only do one queue update at a time (per process). While one batch is being pushed to the objet store, the
following one is being built up, and written as soon as the previous write completes. This can be see in the timings from
the logs of the unit test OStoreSchedulerDatabaseTestVFS/SchedulerDatabaseTest.createManyArchiveJobs/0 here:
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-68" sizeBefore="0" sizeAfter="1" addedJobs="1" waitTime="0.002403" enqueueTime="0.036594"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-68" sizeBefore="1" sizeAfter="100" addedJobs="99" waitTime="0.036547" enqueueTime="0.001629"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-174" sizeBefore="0" sizeAfter="1" addedJobs="1" waitTime="0.000058" enqueueTime="0.061447"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-174" sizeBefore="1" sizeAfter="198" addedJobs="197" waitTime="0.061434" enqueueTime="0.001431"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-174" sizeBefore="198" sizeAfter="200" addedJobs="2" waitTime="0.000840" enqueueTime="0.037259"