[13.x] Add backoff functionality to DB::transaction()

[ziadoz]

This PR adds a backoff parameter to DB::transaction() that allows a delay to be added between deadlock attempts. For example, if there were 3 attempts, there would be 2 delays (between 1-2 and 2-3).

The $backoff parameter can be an integer, array or closure:

// Wait 1,000ms between each attempt...
DB::transaction(fn () => ..., attempts: 3, backoff: 1000);

// Wait 1,000ms between attempt 1-2, then 2,000ms between all subsequent attempts...
DB::transaction(fn () => ..., attempts: 4, backoff: [1000, 2000]);

// Wait a custom amount of time between attempts...
DB::transaction(fn () => ..., attempts: 3, backoff: function ($exception, $currentAttempt, $maxAttempts) {
    return random_int(100, 250) + ($currentAttempt * 100);
});

For non-sqlsrv SQL Server driver connections backoffs won't work, and an exception is thrown if one is configured.

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[tpetry]

That fuether complicates the implementation. In my opinion you should just wrap the transaction into a retry function.

[ziadoz]

@tpetry I had been thinking similarly, which is why I had left it in draft.