fix(connection): ensure pooled connections get released

The PoolConnection is listening for the end event to be emitted to close
the connection, but the connection is not listening to the underlying
end event emitted by the net.Socket.

Author: tommarien

lib/connection.js

@@ -96,6 +96,10 @@ class Connection extends EventEmitter {
       }
       this.packetParser.execute(data);
     });
+    this.stream.on('end', ()=>{
+      // emit the end event so that the pooled connection can close the connection
+      this.emit('end');
+    });
     this.stream.on('close', () => {
       // we need to set this flag everywhere where we want connection to close
       if (this._closing) {