Allow setting formatter for Zend\Log\Writer\Db via config options

[Martin-P]

This PR fixes issue #5881 and makes it possible to set a formatter in Zend\Log\Writer\Db via the config options.

The last line in Zend\Log\Writer\Db::__construct() (line 90) always sets the formatter like this:

$this->setFormatter(new DbFormatter());

This PR has an additional check to see if there already is a formatter set which is provided via the config options. If there already is a formatter, that formatter has precedence over the default formatter. If no formatter is provided via the config options the default formatter Zend\Log\Formatter\Db (DbFormatter in the code above) will be used.

[weierophinney]

Only issue I see with this PR is it is adding additional methods that are not defined in WriterInterface. Considering that the formatter is primarily an internal detail, I think the functionality of getFormatter and hasFormatter should be made protected visibility.

This will primarily have an impact on the test cases, as you'll need to use reflection to alter the visibility for purposes of the tests; alternately, we can rely on known behaviors to verify that the correct functionality is being invoked.

[Martin-P]

I made the methods getFormatter and hasFormatter public on purpose to increase usablity. While debugging I missed the functionality to see if a formatter was set. Also the method setFormatter is public as well. I can alter the visibility and update the tests, but I hope my motivation is convincing enough to keep them public.

[weierophinney]

@Martin-P The problem, though, is that (a) the methods are only visible in this single adapter, and (b) we cannot add methods to existing interfaces without breaking backwards compatibility.

I'll change the visibility during merge, and update the tests.

[weierophinney]

Merged to develop for release with 2.3.0.

[Martin-P]

The methods getWriter and hasWriter are declared in AbstractWriter. All other writers extend AbstractWriter so that shouldn't be a problem? I do see how altering the interface is a BC break. I didn't think of that.