Handle batch inserts

[telezhnaya]

With sqlx, it's not possible to say

sqlx::query("INSERT INTO table VALUES ?", my_vector_of_objects);

It's not even possible to say

sqlx::query("INSERT INTO table VALUES ?", my_object);

You have to put each field of your object into the placeholder

sqlx::query("INSERT INTO table VALUES (?, ..., ?)", my_object.a, ..., my_object.z);

It becomes a nightmare when we are talking about batch inserts.
I've created the solution which builds the correct SQL from pieces, but it leads to the SQL injections.
I also abused fmt::Display for this, which is also not good.
We need to think about a better solution.

[telezhnaya]

1. [Discussion] Dynamic Query Builder Design launchbadge/sqlx#291
2. https://stackoverflow.com/questions/4922345/how-many-bind-variables-can-i-use-in-a-sql-query-in-mysql-5

[telezhnaya]

launchbadge/sqlx#1766

[telezhnaya]

We came up with MySqlOptions, it's the best we can do right now