Fix noneffective static variable cache in log_message()
An improved version of PR #2584, fixes #2583
Merge pull request #2590 from vlakoff/develop

Fix noneffective static variable cache in log_message()
Add a changelog entry for PR #2590 and further optimize log_message()

CI_Log::write_log() already checks the log threshold, so there's no point in doing it in log_message() as well.
[ci skip] Add MD5 checksums to download page (rel: #991)