add Thread model
thread_id is known when creating a message
core Thread and ThreadLookup
manage thread when processing a mail
add tags attribute to Message
tags set for Message and Thread, take 2 random one for testing
threads structure simplest in cassandra, IndexedThread class to complete
create or update index in cassandra and es
a basic Thread.by_user method
AbstractIndex.get didn't work. thread_id in message is given at init
security_level when creating from mail. output api format classmethod
contacts on IndexedThread
security_level managed by MailIndexMessage
keep mail thread_id as external_thread_id
MDA process security_level, random value at this time
Thread.to_api formatting and working by_user and by_id methods
stop mocking Threads route