Overview
----------------------------------------

Make it easier to route log messages based on their topic (e.g. CiviContribute-related logs vs CiviMail-related logs).

Before
------

`Civi::log()` always returns the same instance of `LoggerInterface`, with no
clear way to differentiate logs of different business subsystems.

After
-----

`Civi::log(...)` allows you to optionally request a `LoggerInterface` for a specific theme, e.g.

```php
Civi::log('mail')->error('Failed to connect to SMTP server');
Civi::log('ipn')->warning('Transaction rejected by payment processor');
```

Technical Details
-----------------

A few things going on here:

* Extensions may start using their own logs (`Civi::log('myext')`) without any special effort.
* It is possible to replace or customize specific logs by defining a service `log.CHANNEL_NAME`.
* The `psr_log_manager` is a service. An extension like https://lab.civicrm.org/extensions/monolog/
  can replace the `psr_log_manager` and use the channel-name in its own way.

There is a limitation here in that the list of channels is open-ended.  It
will be impossible to (eg) detect that a log-user has made a typo in the
channel-name.  However, this seems like the better trade-off if the
alternative is that extensions face races during
installation/uninstallation.