If the tower returns CreateSessionCodeAlreadyExists in response to the
CreateSession message from the client, then skip forward a few key
indices until we find one that the server does not return the error
for. This will allow a client to recover after a data loss incident.
This commit upgrades the wtclient package to make use of the new
`AddressIterator`. It does so by first creating new `Tower` and
`ClientSession` types. The new `Tower` type has an `AddressIterator`
instead of a list of addresses. The `ClientSession` type contains a
`Tower`.
