Add null loggers

[cviebrock]

Adds SwatNullExceptionLogger and SwatNullErrorLogger classes: implementations of the loggers that don't actually log anywhere.

Also adds the ability to clear out all the configured exception/error loggers.

This will save some conditional logic in downstream applications.

Note

The changes to composer.lock and SwatDB.php are unrelated to this PR. It looks like Jenkins was erasing the lock file during CI, which meant that a new minor version of PHP-CS would get installed ... and then CI would fail because the formatting rules would have slightly changed.

I erased and rebuilt the lock file (based on current package versions), fixed the one styling issue that CI identified due to the new PHP-CS version, and then updated the Jenkinsfile to not do this again.

[gauthierm]

Clearing logging should not be necessary as we can just not add loggers when we don't want or need them. Logging configuration is typically done in application code. Depending on the framework, we can override the configuration on a case-by-case basis to use different logging configurations.

There also isn't much use for null loggers as we can opt to not add loggers if we don't want logging.