Keyspace strategies were cloned and allocated into a Vec, just for
creating an iterator to references to them in order to construct a
`ReplicaLocator`. It was done due to lifetime issues (in order to send
references to another thread, they must be 'static).
The problem is solved by sending keyspaces to another thread for
constructing `ReplicaLocator` and then moving it back to the calling
thread afterwards. Clones are elided. Hooray!

By leveraging branched variable initialisation, cloning datacenters and
racks is no longer necessary.

This one wasn't particularly hard.

There no reason why the method should be async.

There was a convoluted logic that can be easily expressed using
itertools' `find_or_first()` iterator method.

`get_working_connections()` not only allocated `Vec`s for all nodes, but
it even cloned them to one big `Vec`. At least the latter could be
avoided by returning an iterator over connections.
The new replacement method, `iter_working_connections()`, was put on
`ClusterData` instead of `Cluster`, because it borrows from
`ClusterData`, so lifetime issues would be brought otherwise.
The two uses of `get_working_connections()`, `Session::prepare()` and
`Session::check_schema_agreement()`, were updated to use
`iter_working_connections()`, and `get_working_connections()` was
deleted as not needed anymore.

As `join_all()` accepts any `IntoIterator`, it accepts iterators in
particular. Allocating a `Vec` is unnecessary.