[3.0] Add additional fields to support better mail queueing

[jdarwood007]

This resolves the mail queue needing to handle failed emails better by implementing 3 additional columns for tracking failures.

• This code does not yet make use of the extra column. This is a placeholder should the need arise to add additional changes to the queue after a stable release.
• tries column will hold the number of times this was put back into the queue, if the max is reached, the email is removed from the queue.
• next_try column holds the time we will next try. The select statement is updated to only grab stuff at the current time for before.
• calculateNextTry() sets up the next time we send by a exponential increase based on the number of tries we have done ensuring we increase our delay ever greater.

[Sesquipedalian]

There's no need for a migration step. The upgrader will automatically take care of normalizing the table (meaning, in this case, adding the new columns):

SMF/Sources/Maintenance/Tools/Upgrade.php

Lines 1096 to 1103 in fcd3cc4

	// Ensure the tables are structured correctly.
	foreach (Table::getAll($ns) as $table) {
	$substeps[] = new GenericSubStep(
	name: Lang::getTxt('upgrade_normalizing_table', ['table' => Config::$db_prefix . $table->name], file: 'Maintenance'),
	exec: [$table, 'normalize'],
	);
	}
	}

[jdarwood007]

Should we rely on that or ensure that the upgraded schema is in place and let the catchall be the final cleanup? I had assumed it was the latter and would be our safety catch.

[Sesquipedalian]

If the new table needs to exist in order to perform additional migration steps, then creating it is fine. But if that isn't the case, then leaving it until that final step makes good sense. Why have redundant code?