Call dispatch_semaphore_wait in the main thread

We need to block the main thread, otherwise we'll keep walking and
generating I/O while the transcoders are busy.
Log the number of threads in verbose mode
Use the number of logical cores for -t

As it turns out, processorCount is logical cores, not physical
cores. Some quick experiments show a small but consistent gain
in using processorCount instead of processorCount * 2.
Leftover from the old text