Generate MySQL DELETE queries more amenable to optimizations

This MySQL-specific form seems to be subject to more optimizations
than the standard SQL one.

slick/src/main/scala/slick/jdbc/JdbcStatementBuilderComponent.scala

@@ -464,6 +464,10 @@ trait JdbcStatementBuilderComponent { self: JdbcProfile =>
      b.build
    }

    protected def buildDeleteFrom(tableName: String): Unit = {
      b"delete from $tableName"
    }

    def buildDelete: SQLBuilder.Result = {
      def fail(msg: String) =
        throw new SlickException("Invalid query for DELETE statement: " + msg)
@@ -479,7 +483,7 @@ trait JdbcStatementBuilderComponent { self: JdbcProfile =>
      }
      val qtn = quoteTableName(from)
      symbolName(gen) = qtn // Alias table to itself because DELETE does not support aliases
      b"delete from $qtn"
      buildDeleteFrom(qtn)
      if(!where.isEmpty) {
        b" where "
        expr(where.reduceLeft((a, b) => Library.And.typed[Boolean](a, b)), true)

slick/src/main/scala/slick/jdbc/MySQLProfile.scala

@@ -18,6 +18,7 @@ import slick.sql.SqlCapabilities
import slick.util.{SlickLogger, GlobalConfig, ConstArray}
import slick.util.MacroSupport.macroSupportInterpolation
import slick.util.ConfigExtensionMethods.configExtensionMethods
import slick.util.SQLBuilder.Result

/** Slick profile for MySQL.
  *
@@ -182,6 +183,24 @@ trait MySQLProfile extends JdbcProfile { profile =>
      expr(n)
      if(o.direction.desc) b" desc"
    }

    // Override default DELETE FROM syntax in order to produce a more efficient
    // DELETE query for MySQL.
    //
    // Slick cannot directly handle multi-table DELETEs, i.e., using JOIN or
    // USING, but it can handle subqueries in the WHERE clause of a DELETE.
    // This is good except for the fact that MySQL doesn't know how to
    // optimize such semi-join subqueries to joins in single-table DELETE
    // queries. However, if the DELETE query is a multi-table DELETE, even if
    // on a single table, then something in MySQL kicks in and optimizes the
    // subquery to a more efficient JOIN. Further reading:
    //
    // - http://mysqlserverteam.com/multi-table-trick
    // - https://mariadb.com/kb/en/mariadb/semi-join-subquery-optimizations
    //
    override protected def buildDeleteFrom(tableName: String): Unit = {
      b"delete $tableName from $tableName"
    }
  }

  class UpsertBuilder(ins: Insert) extends super.UpsertBuilder(ins) {