sketch of async resolution strategy

Sketch of implementation for async resolver using threads:

  * program communicates with a resolver thread via a socketpair
  * resolver thread created with first request
  * resolver thread manages a pool of worker threads
    * communicates with workers over socketpairs
    * basically a select() loop
    * if all workers are busy, resolver thread queues the request
  * workers use getnameinfo()/getaddrinfo() to resolve a hostname to IP address string
  * comms are line-oriented
  * program sends a request "ID HOSTNAME\n"
  * resolver dispatches the request to a worker and sends back the result through master sock
  * result comes in the form "ID IPADDRSTRING\n"

Details of thread pool management are in progress.  Startup costs shouldn't be important for resolution, so a clean implementation triumphs.

Currently working on pthread implementation, which ought to be easy to switch to Tcl_Thread
and wrap in a convenient TclObjCommand.

Getting it into the notifier should also be easy (?)

No promises this will be done in a reasonable timeframe; it's mostly a learning exercise for now.

Not thinking hard about the Tcl command interface yet, but expect something like:

  resolver init
  resolver resolve $hostname -command [info coroutine]
  resolver configure -workers

Hopefully won't need deep re-engineering to attach to Notifier (single socket).

How Tcl_SocketObjCommand will make use of this, I don't know.

Requirements:

  * POSIX threads or Tcl_Thread
  * POSIX socketpair
  * getnameinfo/getaddrinfo

Not currently considering:

  * control over getnameinfo params:  fixed at AF_UNSPEC + IPPROTO_TCP
  * I18n Domain Names support