write_dns_event may cause underflow when proxy.config.dns.retries is changed

[udbxtd2008]

Position of the problem:
write_dns_event DNS.cc line 1140: e->id[dns_retries - e->retries] = i;

1. e->retries was assinged with dns_retries in DNSProcessor::getby.
2. The value of dns_retries is combined with "proxy.config.dns.retries".
3. if "proxy.config.dns.retries" is changed to be a smaller value by traffic_ctl， for example:
   /opt/ats/bin/traffic_ctl config set proxy.config.dns.retries 9 -- > 0.
4. [dns_retries - e->retries] will underflow. The memory before id[] will be writen by wrong value.

struct DNSEntry : public Continuation {
int id[MAX_DNS_RETRIES];
...

Solution:
Consider assign dns_retries to DNSEntry in DNSProcessor::getby?

[SolidWallOfCode]

I am looking at this but I've found some other, rather more serious issues. I'll try to work this in as I go.

[bryancall]

We should set proxy.config.dns.retries not be a dynamic config for now.