JSONlog Drupal 8 version

@lorenzs Truly great job you've done here :-)
However, there are several things to be done prior to a release. This is what I stumbled upon - there will turn up more on the way ;-)

Todos - major

1. The module doesn't log at all, except when tested directly via it's admin form - apparantly it doesn't succeed in implementing the D8 equivalent of hook_watchdog.
2. Establishing default site ID doesn't work; the function jsonlog_default_site_id() should fetch the name of the default database via D8 means (not via D7'ish $GLOBALS['databases'], which btw is void in the context).

Todos - minor

1. OOP - the .module file should only contain stuff that absolutely cannot be placed in PHP classes (for instance the settings form should probably extend or negotiate with D8 ConfigFormBase).
2. Coding style - lots of (allright minor) flaws, which an appropriate IDE can fix in whim.
3. module_load_include has been replaced by something else ... loadInclude().

Oops. No wonder it didn't log at my end.

Forgot about severity threshold (default: warning) - I was attempting to log debug messages.

And (more interesting) wasn't aware that the equivalent of D7 watchdog(...)
D8 \Drupal::logger($type)->log(...)
only logs to a single channel - Log API:

D8 - injecting a specific channel
Drupal core registers only one channel in core.services.yml for "system" channel.

dblog was enabled too, at my end.

@lorenzs Great job.

Todos - major (revisited)

1. Yep - it does log.
2. Swell.

Todos - minor (revisited)

1. Yes, I can see that some core modules actually use old-school functional PHP for admin forms. So lets not spend time on that - at least until it gets deprecated or unsupported.
2. Surely a minor issue, didn't stumble upon anything today, except some undocumented return types. I use the same IDE ;-)
3. Right.

Added CHANGELOG.txt

Great as easily accessible tracking. And useful when creating release(note)s.