Support batch insert for pooled sqlite connections and/or make it clear that connection pooling shouldn't be used with SQLite

[tahahawa]

This was used to help reproduce the issue.

https://gitlab.com/tahahawa/discord-markov-bot/blob/eae22b1744348b70b5ceccbe9db148b34352b41b/src/commands/helper.rs

The code:

let conn = sql_pool.get().unwrap();
let _ = diesel::replace_into(messages::table)
    .values(message_vec)
    .execute(&conn)
    .expect("Error inserting values");

Causes:

error[E0277]: the trait bound `diesel::sqlite::Sqlite: diesel::backend::SupportsDefaultKeyword` is not satisfied
   --> src/commands/helper.rs:184:10
    |
184 |         .execute(&conn)
    |          ^^^^^^^ the trait `diesel::backend::SupportsDefaultKeyword` is not implemented for `diesel::sqlite::Sqlite`
    |
    = note: required because of the requirements on the impl of `diesel::query_builder::QueryFragment<diesel::sqlite::Sqlite>` for `diesel::insertable::BatchInsert<'_, models::InsertableMessage, schema::messages::table>`
    = note: required because of the requirements on the impl of `diesel::query_builder::QueryFragment<diesel::sqlite::Sqlite>` for `diesel::query_builder::InsertStatement<schema::messages::table, diesel::insertable::BatchInsert<'_, models::InsertableMessage, schema::messages::table>, diesel::query_builder::insert_statement::Replace>`
    = note: required because of the requirements on the impl of `diesel::query_dsl::load_dsl::ExecuteDsl<diesel::r2d2::PooledConnection<diesel::r2d2::ConnectionManager<diesel::SqliteConnection>>, diesel::sqlite::Sqlite>` for `diesel::query_builder::InsertStatement<schema::messages::table, diesel::insertable::BatchInsert<'_, models::InsertableMessage, schema::messages::table>, diesel::query_builder::insert_statement::Replace>`

Current fix:
Change &conn to &*conn

Setup

Versions

• Rust: 1.28.0
• Diesel: 1.3.2
• Database: SQLite with R2D2 pooling
• Operating System Arch Linux

Feature Flags

• diesel: ["sqlite","r2d2"]

Checklist

• I have already looked over the issue tracker for similar issues.
• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)