Check connections when get_client_slow fails

[gu0keno0]

Fix for #367

[chvck]

This will end up attempting to rebuild connections when get_client_slow returns Shutdown. I think that the correct approach here might be to not check for a cached error at all in get_client_slow, and to call

self.check_connections().await;
self.rebuild_fast_map().await;

either at that point or do something similar when client creation fails within start_new_client_thread so that we proactively attempt to have a new client ready. What do you think?

As an aside between our developer preview and GA release I'm hoping to rewrite the kvclient, kvclientpool, and kvclientmanager stack as it's too complicated and difficult to understand (especially failure cases)

[gu0keno0]

Fair point. I'll look into the two approaches and update the PR.