2 models for thread lookup using recipient or external thread id
add position to MessageLookup model
add external_thread_id to Message model
no more MessageLookup, use ThreadMessageLookup and move offset and position properties to Message
date instead of position for Message ordering.
date field is not part of clustering so can not be use for order by query
add message_id in ThreadMessageLookup
Merge pull request #5 from gdchamal/thread_lookup

Thread lookup