Added a first version of Cassandra MAM backend.
Added query_refs_count for mod_mam_muc_ca_arch.
Added new book filter_local_packet. Fixed {badarg,[{ets,member,['ejabberd_users_muc.localhost',{<<alicesroom>>,<<muc.localhost>>}],[]} during sending a MUC message.
Added a special version of mod_mam_cache_user for MUC.

Added an empty stop_server function.
Added mod_mam_odbc_server_user: archive id assignor for multiple hosts.
Added unique constraint for mam_user and mam_server_user tables.
Deleted MAM_ARCHIVE_PRESENCE. Splited is_complete_message into several ones.
Added seestar into release dependencies.
Fixed wrong function name.
Added prepare_user_id/1 for mod_mam_ca_arch.
Added mod_mam_muc_ca_arch:select_worker/2.
Merge branch 'arc-mam-mods' into arc-mam-ca
Added mod_mam_muc_ca_arch:prepare_room_id/1.
Added servers config variable for mod_mam_muc_ca_arch.
Select worker based on archive id in mod_mam_muc_ca_arch.
Deleted prepare_room_id.
Added mod_mam_con_ca_arch.
Added mam_con_user table. for tracking conversations.
Removing messages from mod_mam_con_ca_arch.
Choosed a better strategy of cleaning conversation archives.