…nce EOL of 1.8.7 support
:ignore could be taken to mean 'ignore the chroot portion of the connection string' so to avoid confusion, change to :do_nothing, which hopefully gives users the impression "We won't do anything regarding the existence or non-existence of the chroot path." (Which was a contender for the option name, but was ruled too verbose by the East German judge).
Still mixed into ZookeeperCallbacks::WatcherCallback, but now at least looks like it's part of ZK, and provides useful documentation.
allow the user to specify at registration time what kind of events they want the block to receive, rather than always having to do (for example) `event.node_changed?`
also some extensions cleanup (to core_ext.rb)