Implement $protocol parameter in Config base_url() and site_url() methods

Let's keep the implementation logic in one place.
Improves 2023c3d05b042cf1322286d69557c2b8bf3bd8d5.
Remove redundant changelog entry
Merge pull request #2699 from vlakoff/url-functions

Implement $protocol parameter in Config base_url() and site_url() methods
Fix CI_DB_query_builder::_merge_cache() triggering E_WARNINGs

Kudos to kakysha for noting the bug & providing initial fix in PR #2698