Replace WorkerMaster with per-worker listeners.

- Fixes a bunch (it turns out) async-related bugs in
  WorkerMaster.maybeDestroy().
- Gets rid of HANDSHAKE message since each worker has its own dedicated
  socket.
- Rename WorkerImpl to Worker, as all implemnetation details can now be
  hidden. Some were formerly exposed for use by WorkerMaster.

lib/webworker-child.js

@@ -108,9 +108,6 @@ var ms = new wwutil.MsgStream(s);
 
 // Once we connect successfully, set up the rest of the world
 s.addListener('connect', function() {
-    // Perform handshaking
-    ms.send([wwutil.MSGTYPE_HANDSHAKE, process.pid]);
-
     // When we receive a message from the master, react and possibly
     // dispatch it to the worker context
     ms.addListener('msg', handleMessage);

lib/webworker-util.js

@@ -18,9 +18,8 @@ var STACK_FRAME_RE = /.* \(?(.+:\d+:\d+)\)?$/;
 
 // Symbolic names for our messages types
 exports.MSGTYPE_NOOP = 0;
-exports.MSGTYPE_HANDSHAKE = 1;
-exports.MSGTYPE_ERROR = 2;
-exports.MSGTYPE_CLOSE = 3;
+exports.MSGTYPE_ERROR = 1;
+exports.MSGTYPE_CLOSE = 2;
 exports.MSGTYPE_USER = 100;
 
 // Is the given message well-formed?