2.0.0.0-dev43

* Implemented functional limitation that restricts max number of catalog products in the system
* Implemented cache backend library model for MongoDB
* Converted some more grids in backend from PHP implementation to declarations in layout
* Removed `app/etc/local.xml.additional` sample file, moved detailed description of possible configuration options to documentation
* Refactored `Mage_Core_Model_EntryPointAbstract` to emphasize method `processRequest()` as abstract
* Moved declaration of functional limitations to the nodes `limitations/store` and `limitations/admin_account`
* Bug fixes:
  * Fixed JavaScript and markup issues on product editing page in backend that caused erroneous sending of AJAX-queries and not rendering validation messages
  * Fixed issues of application initialization in cases when `var` directory doesn't have writable permissions. Writable directories are validated at an early stage of initialization
  * Fixed array sorting issues in test `Magento_Filesystem_Adapter_LocalTest::testGetNestedKeys()` that caused occasional failures