Rebase onto upstream 0.4.2 release with these additional fixes:

• Fix spammy log when read times out.

• Fix iteration over leaves and add unit tests.
• Work around bug in etcd where it ignores prevIndex if prevExists is set.
• Tighten up some except: clauses to only catch exceptions that are expected.
• Move cluster ID check inside except block so that we can do _ = response.data. This forces the data to be loaded before leaving api_execute(). Without this change, urllib exceptions can be deferred until we do _result_from_response(), which bypasses the exception handling in api_execute().

Upstreaming of these fixes is tracked under jplana#127