Add an option to disable parallelism in v8

[garak]

See also discussion #4083 for a reference.

In version 8 (v8.0.0-alpha.2 when I'm writing this), it was introduced a new mechanism to enforce parallelism. This mechanism requires additional extensions to be installed in your PHP environment tho, and it's enabled by default.

My request is to add at least an option to disable it, to ease the upgrade from v7.
In my opinion, the best way would be to enable it only explicitly, to minimize BC.
Still, it's also acceptable to have just an option to disable it (as soon as it's documented in the upgrade path).

[garak]

I just tried with the stable v8, and I see the problem disappeared, so I guess that it was fixed.
This can be considered solved from my point of view.

[antonmedv]

Nice! Thanks for feedback!