cell filter pages rendering speed-up

The MAX_UNFILTERED_ENTITIES config variable used to prevent rendering
large sets of unfiltered results. This was helpful in reducting http
traffic and rendering time in browser, but the database fetching of all
entities of particular realm was performed anyway.

This change disables the database fetching of entities if there is no
filter set, and the resulting set size is greater than
MAX_UNFILTERED_ENTITIES config variable value. This allows to quickly
access to the filtering portlet and add/delete tabs on heavy pages
(objects, racks, IP nets).

getEntitiesCount: retrieves the count of enities of specified realm
renderLocationFilterPortlet: check if $_SESSION['locationFilter'] is set
renderRackspace: try to call renderEmptyResults 2 times: to disable
	fetching and to disable displaying
renderDepot: idem
renderIPSpace: idem,
	also the bug if fixed: 'ipv4net' realm was passed to the
	renderCellFilterPortlet even on the 'ipv6space' page