#6622 Logger refactoring: move to PSR-3 compatibility

- keep the log message as string
- exceptions are logged in context under the "exception" key
- Piwik\Error objects are replaced by \ErrorException (logged the PSR-3 way)
#6622 Logger refactoring: removed the "Log.getAvailableWriters" event

Adding new log backends will later be possible by configuring Monolog handlers
#6622 Logger refactoring: moved all the log config to DI config