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namespace into the normal dlib:: namespace so that code which works
with both sparse and dense vectors is more cohesive.

thread_pool.  Previously, each separation oracle call was dispatched
to a thread individually.  This is inefficient when there are a whole
lot of samples (and thus separation oracle calls which need to be made).
So now entire batches of separation oracle calls are dispatched to
each thread.  This minimizes the thread switching and synchronization
overhead.

This only happens if the user implements the separation oracle wrong.  In this
case I had the code setup to force the risk to zero but this just ends up
obscuring the fact that the user's separation oracle is incorrect.  So I'm
removing this check.  This should make debugging separation oracles a little
easier.