A nice creation by @sgiehl

@iquito thanks for the tip, it is a better solution!

Let's see if some test will fail as a result.

in particular adding instruction to restart web server after having updated piwik.
This should help limit number of email reporting errors, as many of these errors are caused by OPcache being not updated.

 (not sure why this has changed in last couple days, no related code was changed)"

Optimizations for import_logs.py Fixes #5314

Kuddos for nice pull request! We hope to see more contributions from you in the future :)

As suggested by cbay, the cache key can be a tuple instead of a string
concatenation.

Fallback to a non cached dates when OrderedDict is not available.
It can occur with python < 2.7 and Pypi OrderedDict is not installed.

cbay reported that set comprehension was available only in python 2.7+.
This patch fixes the syntax to keep backward compatibility with python 2.6.

check_methods are called twice for each hit. The first ones are sufficient to
decide if the hit should be excluded or not.

Fix .PHP Fatal error:  Call to a member function getStylesheet() on a non-object in /home/travis/build/piwik/piwik/core/AssetManager/UIAssetFetcher/StylesheetUIAssetFetcher.php on line 59
https://travis-ci.org/piwik/piwik/jobs/26917086

Prevent notice PHP Fatal error:  Call to a member function getPluginName() on a non-object in /home/travis/build/piwik/piwik/core/Twig.php on line 49

refs #5301 make sure to pass the instance to ScheduledTasks and not the className. Otherwise it is called statically. The disadvantage of this way is all task will be stored under the name "Tasks.$methodName" which can lead to naming collisions very quickly if it is not already the case. I am wondering why we keep only the last part of the class name and not the full namespaced class name here: https://github.com/piwik/piwik/blob/master/core/ScheduledTask.php#L105 ? Keeping the namespace would fix the naming collision. If we change to full namespace it could be possible a scheduled task is not executed once as the name changes