Specify queue concurrency via commandline switch

[ericsoco]

Throttling requests via queue-async concurrency would be a nice alternative/complement to the --delay switch. Maybe could be specified via --concurrency <N>. Will see if I can find time to add it....lib works great, thx @veltman!

[veltman]

That would be nice, the only snag would be that the logging of rows in verbose mode could wind up out of order since those events fire in whatever order the calls complete. That might be OK since it's really for debugging purposes anyway, but in that case might need to include "Row #X: " in the output to avoid confusion.