v2.6.6

• [Fix] Make Taggable::Tags safe to read while another thread adds or removes tags. On JRuby a concurrent read could raise NullPointerException.
• [Fix] Make Taggable#tags return one Tags per object when several threads call it first. On JRuby each thread could get its own Tags, and tags were lost.
• [Fix] Make Notifications#register_event safe while other threads call #instrument. On JRuby a concurrent registration could make a registered event look unregistered or dispatch it without its method.
• [Fix] Make the Monitor event name cache thread-safe for a namespace or Symbol event ids, so on JRuby concurrent #instrument calls no longer corrupt it.