This is not specific to authentication, and allows plugins greater flexibility in what events they are interested in. It also adds message handling, and $CONTROL handling.
Tests for: * MOSQ_ACL_SUBSCRIBE acl check * MOSQ_ERR_PLUGIN_DEFER with 2 auth-plugin * Binary compatibility with v2 of auth-plugin Signed-off-by: Pierre Fersing <pierre.fersing@bleemeo.com>