Throttle Hive splits discovery

[BenoitHanotte]

This is a work in progress to throttle the hive split discovery.

If the value of hive.max-outstanding-splits is greater than 0, then HiveSplitSource will use the ThrottledAsyncQueue to enqueue the splits, which throttles the returned batches to release at most that amount of splits per seconds to the consumer, thus throttling the split discovery mechanism for hive.

Note that hive.max-outstanding-splits needs to be more than query.min-schedule-split-batch-size otherwise BufferingSplitSource will buffer the throttled splits and assign them all at once, cancelling the "smoothing" effect of throttling the splits.