cluster: remove handles when disconnecting worker

cluster: remove handles when disconnecting worker

Due to the race window between the master's "disconnect" message and the
worker's "handle received" message, connections sometimes got stuck in
the pending handles queue when calling `worker.disconnect()` in the
master process.

The observable effect from the client's perspective was a TCP or HTTP
connection that simply stalled.  This commit fixes that by closing open
handles in the master when the "disconnect" message is sent.

Fixes: https://github.com/nodejs/node/issues/3551
PR-URL: https://github.com/nodejs/node/pull/3677
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James M Snell <jasnell@gmail.com>