Explicitely mention this is a public

Signed-off-by: Michal Čihař <michal@cihar.com>
Do not hardcode mysqli in the interface library

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove never used parameter of cacheGet

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove never used parameter of cacheExists

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove never used parameter of cacheSet

Signed-off-by: Michal Čihař <michal@cihar.com>