MariaDB: add support for RETURNING clause in DELETE statements

[capcom6]

This PR adds support for RETURNING clauses in DELETE statements specifically for MariaDB databases. MariaDB has supported the RETURNING clause in DELETE statements since version 10.0.5.

Changes:

• Introduce feature.DeleteReturning because the existing feature.Returning is used in UpdateQuery, but MySQL/MariaDB don't support RETURNING clause in UPDATE statements
• Add feature.DeleteReturning to all dialects already having feature.Returning for backward compatibility
• Add feature.DeleteReturning to mysql dialect for MariaDB 10.0.5 and later
• Replace feature.Returning with feature.DeleteReturning in DeleteQuery
• Add check for feature.DeleteReturning in DeleteQuery.Returning method because previously, Returning could be used for the mysql dialect but didn't receive any error or result

Example usage:

db.NewDelete().Model(new(Model)).WherePK().Returning("*")

Notes:

• MariaDB doesn't support RETURNING clause for multi-table deletes but I haven't add any check for that because I don't know is any other database supports this.

Discussion:

• Not directly related to this PR: There is feature.InsertReturning, but InsertQuery.tryLastInsertID checks feature.Returning. Is this correct?
• After this PR, feature.Returning will be used only in UpdateQuery. I propose renaming it to feature.UpdateReturning to clarify its usage. This renaming has not been implemented yet.

References:

• MDEV-3814
• MariaDB 10.0.5 Release Notes

[j2gg0s]

For dialects that do not support DELETE RETURNING.
Before: We remained slient on misuse and ignore it when append query.
After: We return error on misuse.

I dont think current solution is correct, but i dont think we should break it,
as that might casue potential failures in upper-level application.

[capcom6]

On the other hand, I was very discouraged when I tried to use Returning() with DeleteQuery and received an empty result. I propose adding a note to the method's documentation, something like: "If the dialect does not support DELETE RETURNING, an empty result will be returned without an error."

[j2gg0s]

I dont have a strong preference, let @vmihailenco decide.

[vmihailenco]

I think returning an error might be annoying in some cases, but way easier to debug...