Allow multiple GeoIP filters (fixes bug introduced in #990)

This fixes LOGSTASH-1842 by using a per-object thread-local key name.

Previously (via #990), all geoips would compete over the same
threadlocal key name :geoip. Now the threadlocal key is
'geoip-<the geoip filter object id>' which allows each instance of geoip
to use a unique key and not share with another filter instance.
Merge pull request #997 from jordansissel/fix-thread-local-geoip

Allow multiple GeoIP filters (fixes bug introduced in #990)