Ensure that calling Next twice in a row without first calling Reset is
safe when the iterator is at the end of its list. Also alter the
towerListIterator to call Reset after hitting an error on Next.
In this commit, the bug demonstrated in the previous commit is fixed.
The locking capabilities of the AddressIterator are used to lock
addresses if they are being used for session negotiation. So now, when a
request comes through to remove a tower address then a check is first
done to ensure that the address is not currently in use. If it is not,
then the request can go through.
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`.
This addresses a potential panic when a tower has one of its candidate
sessions chosen, but its only reachable address was removed by a
user-initiated RPC before the fact.
