The API of `Registry` has been modified as follows:
 - `Registry(name)` returns the registry of the given database
 - `Registry.new(name)` forces the creation of a new `Registry` object
 - `Registry.delete(name)` discards the given registry
 - `Registry.delete_all()` discards all registries
 - `registry.setup_signaling()` initializes the signaling for `registry`
 - `registry.check_signaling()` returns an up-to-date `registry`
 - `registry.signal_registry_change()` notifies that `registry` has changed
 - `registry.signal_caches_change()` notifies potential cache invalidation

The db_name parameter passed in command-line (-d) or in configuration file,
supports a list of database names separated by commas, in order to
preload them. The WorkerCron processes will also respect this list and only
connect to the specified database(s).

As of Odoo 9 the server also automatically initializes a new database
if the one passed via `db_name` does not exist or is not initialized.
This code did not properly handle multiple comma-separated value,
and created an unitialized database named `a,b` when db_name was set
to `a,b`.

Fixes #13079
Closes #13094

As atexit function are inherited by subprocess, the pidfile was always
deleted when the first worker (http or cron) died. Now, only the
process that created the pidfile will delete it.

- Preserved explicit 3rd-party copyright notices
- Explicit boilerplate should not be necessary - copyright law applies
  automatically in all countries thanks to Berne Convention + WTO rules,
  and a reference to the applicable license is clear enough.