mysql: close last_stmt for non-cacheable statements

[gwy15]

Since we disabled mysql_async's caching mechanism and in that way it requires manual close the statement, we need to close all prepared stmts before dropping it.

From my understanding, and correct me if I'm wrong, as long as statements are cacheable, we will never drop them. So we only need to drop non-cacheable stmts. This PR implements this.

Concern

Is it better to close prepare statements right after execution or close before the next execution of non-cacheable statement (the implementation)?

close #26

[gwy15]

diesel seems to guarantee that For queries which are unsafe to cache, ... will be prepared and immediately released after use

In that way I think last_stmt should be dropped and Queryable::close-ed right after executing callback. But the returned R has lifetime 'conn and Rust think it borrowed conn and last_stmt which makes it impossible to use conn and last_stmt again...

[weiznich]

Thanks for opening this PR. I've filled #29 with a more complete solution. I will likely merge the other PR.

[weiznich]

Closed by #29