Refactor zone dumping code to use netmgr async threadpools

Refactor zone dumping code to use netmgr async threadpools

Previously, dumping the zones to the files were quantized, so it doesn't
slow down network IO processing.  With the introduction of network
manager asynchronous threadpools, we can move the IO intensive work to
use that API and we don't have to quantize the work anymore as it the
file IO won't block anything except other zone dumping processes.