Fix some bugs in the pinging and ID checking.

Raise an exception if the node's ID has changed. This means the node's
returned results will be ignored.

The KTable replaceStaleNode now adds the new node if there's space even
if the old one isn't found. This is due to the node ID having changed
resulting in instantly removing the node from the routing table.

The handler for the ping result in insertNode was mistakenly taking the
Failure returned as a result as a node and passing it to replaceStaleNode.