revisit security method for mysql

[jenseng]

see instructure/canvas-lms#179

hair_trigger never explicitly sets SQL SECURITY for mysql, so it defaults to DEFINER. you can specify an alternate definer with the security method, but you cannot specify :invoker (as you can for postgres). we should probably support this and make it the default.

this may help for scenarios where people are using binary logging for mysql but do not have log_bin_trust_function_creators enabled. some extra investigation should be done to see what else we can do there.

see also https://forums.aws.amazon.com/thread.jspa?messageID=181915