Move _SynchronousQueue into another file

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diffs better.
Rename _SynchronousQueue to _UnyieldingQueue

Allow server uses of LocalCollection to prefer _UnyieldingQueue. This
will mean that, as long as their observeChanges callbacks do not yield,
calls to insert/update/remove will not yield either.
Optionify the barely-used LocalCollection name arg
More comments about our sort inconsistency
Refactor OplogObserveDriver to use LocalCollection

This should be a good starting point for implementing limit/sort/skip.
Merge branch 'oplog-localcollection' into devel