I think just ignoring ZOO_SESSION_EVENT is fine.

How about this?:
* Handle session events in global watcher (which is set via `zookeeper_init`): as in pull-req #776: https://github.com/jubatus/jubatus/pull/776/files#diff-b0bd84100e7738d4f56f8102ebbaaa11L500
* Handle node-specific events (events other than ZOO_SESSION_EVENT) in this watcher.