This extends threadpool slightly so it can bind enqueued tasks to a
secure streams handle as well as a straight wsi.
Either the .wsi must be set as before, or the .ss handle if you are
using secure streams, when enqueuing a task on the taskpool.
A couple of other helpers get ss-aware wrappers if LWS_WITH_SECURE_STREAMS
Although threadpool was originally designed for server (gitohashi)
actually it's also fine working with client wsi / Secure Streams,
if you have a situation a client connection is associated with heavy
processing.
