[nicolas-grekas]

Stacked on #2617: review the last commit.

Shared state for background workers, the vars half of #2287. A worker declares the namespaces it publishes, and readers name the namespace, never the worker:

worker {
	file config.php
	background
	vars redis feature_flags
}

// the background worker, before its first tick()
$handle->setVars('redis', ['dsn' => 'redis://cache:6379']);

// anywhere
$redis = frankenphp_get_vars('redis');

1. A namespace has one publisher per scope, the reader's php_server first, then the global workers, and Validate() refuses conflicts; a worker publishing vars runs a single thread. Between two requests, a worker script reads as the request before it did.
2. Every declared namespace is published before the first tick(), which the server start waits for, so requests never wait for a snapshot.
3. Snapshots belong to the worker, not to a run: a restarting worker keeps serving its last ones. They own their memory, so a reboot of the engine can't leave them dangling.
4. Values are null, scalars, arrays or enums, and references in arrays are followed. A snapshot that doesn't fit in what is left of the reader's memory_limit throws instead of being copied.

Left out on purpose: the unchanged-data skip of #2287, until there are numbers for it. The per-request cache is #2827.