Refactor the dns_geoip API to use ISC_THREAD_LOCAL

Refactor the dns_geoip API to use ISC_THREAD_LOCAL

Previously, the dns_geoip API used isc_thread_key API for TLS, which is
fairly complicated and requires initialization of memory contexts, etc.
This part of code was refactored to use a ISC_THREAD_LOCAL pointer which
greatly simplifies the whole code related to storing TLS variables, and
creating the local memory context was moved to named and stored in the
named_g_geoip global context.