Retry requests when the worker has the connection close

When a tcp_closed or ssl_closed message is handled while ibrowse is
trying to send a request to a worker, ibrowse would incorrectly return a
req_timedout message, when, in fact, the request had not even been
handled.

This patch attempts to avoid that race condition by signalling tcp or
ssl closed messages back to ibrowse differently than a normal exit.
Merge pull request #100 from Vagabond/adt-connection_close

Retry requests when the worker has the connection close