Semi-sync slave plugin assumes only one master

Current semi-sync slave plugin implementation assumes that MariaDB has only one master and only one slave IO thread. As an example it has global variables semi_sync_need_reply and rpl_semi_sync_slave_status which should be per-master. Also I'd think object repl_semisync_slave should be per-master too.
As general MariaDB design is done for multi-master replication, semi-sync slave plugin should be adjusted for that too.