Add trampoline around iocompletionport_createthreads()

Add trampoline around iocompletionport_createthreads()

On Windows, the iocompletionport_createthreads() didn't use
isc_thread_create() to create new threads for processing IO, but just a
simple CreateThread() function that completely circumvent the
isc_trampoline mechanism to initialize global isc_tid_v.  This lead to
segmentation fault in isc_hp API because '-1' isn't valid index to the
hazard pointer array.

This commit changes the iocompletionport_createthreads() to use
isc_thread_create() instead of CreateThread() to properly initialize
isc_tid_v.