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
Fixes #5182 - Adds file_exists results cache in ElggViewService
moved session db code into class
Fix activity page titles
Fixes #4913: Admins can view any user's activity stream
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
Fixes #4813 removes topbar logo
Fixes #4376 fixed incorrect use of bypass in 2 elgg view functions