Update anti_ddos_challenge.lua

Remove dependancy for ngx.re.gsub tests come back string.gsub is fast enough and performs better overall.
Improved cache logs to make more readable/understandable
Added extra details to Range, WAF and blocking logs
Remove un-needed custom command checks
Fix incase user does not want to use shared memory zones the function to obtain users real ip was not present added.

Author: C0nw0nk