added session to service provider
using ElggSession in sessions lib
replaced calls to session global with ElggSession in engine
session_ functions replaced by session object
added storage and tests
deprecated global $SESSION
not running PHP session native storage tests on Travis
pulled more session code into ElggSession
moved session db code into class
Fixes #3481. Data moved to GUIDs instead of dates.
Migrated to Elgg_EntityDirLocator. Updated simpleTest tests.
Fixed upgrade for data dir.
Merge pull request #5111 from brettp/data_to_guids

Use GUIDs instead of dates for data dir.
Adds testing README
Refs #5199 - Adds additional info to locate output start in case of headers already sent exception
Fixes #5064 moves db_upgrade() back to upgrade lib
added elgg_get_session()
Merge pull request #5178 from cash/session

Session Rewrite
removed deprecated use of $vars['url']
Fixes #5110 undeprecates the db functions