fix infinite loop with kafka admin

[hackaugusto]

Port of dpkp#2194

An infinite loop may happen with the following pattern:

self._send_request_to_node(self._client.least_loaded_node(), request)

The problem happens when self._client's cluster metadata is out-of-date, and the
result of least_loaded_node() is a node that has been removed from the cluster but
the client is unware of it. When this happens _send_request_to_node will enter an
infinite loop waiting for the chosen node to become available, which won't happen,
resulting in an infinite loop.

This commit introduces a new method named _send_request_to_least_loaded_node which
handles the case above. This is done by regularly checking if the target node is
available in the cluster metadata, and if not, a new node is chosen.

Notes:

This does not yet cover every call site to _send_request_to_node, there are some
other places were similar race conditions may happen.
The code above does not guarantee that the request itself will be sucessful, since
it is still possible for the target node to exit, however, it does remove the
infinite loop which can render client code unusable.

[ivanyu]

LGTM to the best of my knowledge of the library.