Router: introducing the PORT_ACK message.

The PORT_ACK message is the router's response to the application's NEW_PORT
message.  After receiving PORT_ACK, the application is safe to process requests
using this port.

This message avoids a racing condition when the application starts processing a
request from the shared queue and sends REQ_HEADERS_ACK.  The REQ_HEADERS_ACK
message contains the application port ID as reply_port, which the router uses
to send request data.  When the application creates a new port, it
immediately sends it to the main router thread.  Because the request is
processed outside the main thread, a racing condition can occur between the
receipt of the new port in the main thread and the receipt of REQ_HEADERS_ACK
in the worker router thread where the same port is specified as reply_port.

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