This was causing flakiness of acceptance test ReinstallFromScratchCest.php:reinstallFromScratch
and also potential issue for production.
In Activator::deactivate we remove MailPoet's DB tables. This takes some time.
In case there is another request during the deletion it may detect some tables are missing and trigger activation
and we end up creating MailPoet tables and removing them at the same time.
This commit adds locking to deactivate method so that any other attempt for activation ends up with an error.
[MAILPOET-4274]
