Adjust the fillcount and freemax for dns_message mempools

Adjust the fillcount and freemax for dns_message mempools

According to the measurements (recorded on GL!5085), the fillcount of 2
for namepool and fillcount of 4 for rdspool can fit 99.99% of request
for tested scenarios.

This was discovered by perf recording the single second recursive test
using flamethrower where the initial malloc lit up like a flare.