Fix: Invoice number race condition

[alexmigf]

closes #619

This PR attempts to fix the race condition issue spotted using Stripe plugin.

It changes the lock time to 60 seconds, to avoid expiring before the code being finished, and now also adds a lock to the email attachment function as well, which is the primary way of getting duplicated numbers.

[alexmigf]

@MohamadNateqi check now

[MohamadNateqi]

LGTM!

[MohamadNateqi]

All good!

[MohamadNateqi]

The last commit was great.

What do you think of checking that setting here and sending an empty array if it's not checked, instead of adding that in the Semaphore class? Then the log() method can only check the emptiness of that property.

woocommerce-pdf-invoices-packing-slips/includes/class-wcpdf-main.php

Line 174 in 842bc34

$lock = new Semaphore( $this->lock_name, $this->lock_time, array( wc_get_logger() ), $this->lock_context );

[MohamadNateqi]

Perfect!