Implemented: Support for static cache handler system

Implemented: Support for static cache handler system. Added StaticCache enabled check before performing executeActions method to mirror kernel implementation

Hello,

First, Thank you again for merging our last static cache related pull request! You continue to make working with your extension so great!

The problem
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We recently implemented static cache support for a site and found that content imported with sqliimport did not get static cache created when clearing the view cache via the 'sqliimport_cleanup' cronjob.

After some additional research we found that static cache was not being created because when 'sqliimport_cleanup' cronjob runs to clear the view cache and run the 'eZStaticCache::executeActions()' method we realized we were actually using a StaticCacheHandler and not the eZStaticCache class at all so calling 'eZStaticCache::executeActions()' method due to implementation specific internals of this method would not trigger the storage of delay static cache requests. 

See the data stored in our CustomStaticCache::$actionList is not available to the eZStaticCache::$actionList class variable because it is a private variable of a different class, so our previous pull request is not quite enough to properly cover the most common static cache use cases like the use of a StaticCacheHandler.

The solution
=========

The solution to the above problem was to replace the line of code used by the 'sqliimport_cleanup' cronjob part to call 'eZStaticCache::executeActions()' method with '$staticCacheHandlerClassName = eZINI::instance( 'site.ini' )->variable( 'ContentSettings', 'StaticCacheHandler' ); $staticCacheHandlerClassName::executeActions();'.

Which executes the code to store the handler class's delayed requests to update static cache. Without this change the requests to delay static cache updates are never stored and thus never able to performed. For us this was a big negative feature creating a gaping hole in our otherwise perfect static cache setup.

Closing
======

Please review this pull request. 

This change makes it possible for us to continue to sqliimport extension and StaticCacheHandler setting and we would really appreciate it if we did not have to maintain a patch to this extension in a fork.

Please let us know what you think.

Thank you for your continued support!

Cheers,
Brookins Consulting