Twig should not rely on loading php from shared file system

Problem/Motivation

As #2547827: PhpStorage: past, present and future documented, currently Drupal 8 can't be used on multiple webheads without writing files to a shared filesystem. This is undesirable because a shared filesystem at best is slow at worst doesn't exist at all if stream wrappers are used to store upload files on some mass file hoster.

Proposed resolution

1. Use the extension classes + mtime to invalidate template files, once any of those change. This is done using a hash of those values

The following solutions have been proposed:

1. Add the value of a distributed atomic counter to the file name of the local file. When invalidating the storage, increase the counter. Drawback: it requires an atomic counter query for every read. By default this comes from the database which is a performance hit.
2. Use the cache as phpstorage in such a way that included files can still be opcode cached. Drawback: since the name of the wrappers that can be opcode cached is hardwired in PHP 5.5-7.0 this is necessarily a hack. We do not want to add hacks to core so this is moved to contrib.
3. Do not solve the problem in general, just make core work. Change the container to not use phpstorage at all. For Twig add a hash of the things the compiled code can vary on (core version, deployment identifier, twig extensions) to the file name. Drawback: every web frontend needs a local writable disk and also each will need to compile every Twig file for itself which is a small waste of effort (not too big). These are inherent to this solution and are not fixable. The solution in the previous point does not use a disk at all and it's possible to change it via some kind of locking to have only one process building.

Remaining tasks

User interface changes

API changes

Data model changes

Beta phase evaluation