Ability to disable key tracking

Fix #1550
Updated CHANGELOG
Fix fatal typo in phalcon_mvc_router_group_add_helper()
Fix #2071
Test case
Merge pull request #2075 from sjinks/memcached

Ability to disable key tracking for Memcache/Libmemcached backends
Merge pull request #2076 from sjinks/router_typo

Fix fatal typo in phalcon_mvc_router_group_add_helper()
Merge pull request #2077 from sjinks/issue-2071

Fix Array to string conversion in Phalcon\Http\Response::redirect()