Reduce freemax values for dns_message mempools

It was discovered that NAME_FREEMAX and RDATASET_FREEMAX was based on the NAME_FILLCOUNT and RDATASET_FILLCOUNT respectively multiplied by 8 and then when used in isc_mempool_setfreemax, the value would be again multiplied by 32.

Keep the 8 multiplier in the #define and remove the 32 multiplier as it was kept in error. The default fillcount can fit 99.99% of the requests under normal circumstances, so we don't need to keep that many free items on the mempool.

Closes #2398 (closed)