Ref: trilogy-libraries#105

To close a prepared statement you need to have access to the connection
that created it.

But in managed languages like Ruby, the obvious thing to do is to
close the prepared statement when the associated object is garbage
collected.

But the order in which objects are garbaged collected is never guaranteed
so when freeing a statement the connection might have been freed
already and it's hard to detect.

We checked how libmysqlclient handles it, and each `MYSQL_STMT` has
a reference to its `MYSQL` (connection), and the connection keeps a
doubly linked list of the statements it created.

When a statement is closed it's removed from the list, when the connection
is closed, all the connection references are set to NULL.

We implemented exactly the same logic here.

Additionally, prepared statement can only be used with the connection
they were created from. As such having all the `trilogy_stmt_*` function
take a connection isn't great for usability. So this change opens the
door to only taking a `trilogy_stmt_t *`.

Co-Authored-By: Adrianna Chang <[email protected]>