Don't call `_config_nodes` when another `_config_nodes` is in progress
If we call `get_node` twice before starting IOLoop, `_config_nodes` will be called twice, and therefore subsequent `_config_nodes` will be called twice every 30-seconds.
Fix wrong assumption about synchronous error while connecting to an unavailable node
Merge branch 'patches' of https://github.com/tahajahangir/mongotor into tahajahangir-patches
Merge branch 'tahajahangir-patches'