rename multithreaded flags and set defaults

restore:
--numInsertionWorkersPerCollection
--numParallelCollections=4
--batchSize(hidden, default 10k)
--osThreads (hidden, autodetect to max)

import:
--numInsertWorkers
--numDecodingWorkers (autodetect)
--batchSize (hidden, default 10k)
--osThreads (hidden, autodetect to max)

dump:
--numParallelCollections=4
--osThreads (hidden, autodetect to max)