fix regressions in input/gelf

- fixed regression that caused 'gelf listener died' on chunked messages
- moved client ip of the sender from `event['host']` to `event['source_host']` since this will overwrite the (probably hostname) from the GELF message with an IP address. The hostname is thus lost and would require resolving in a logstash filter or by modifying upstream GELF clients. This is sort of a regression in that previous versions of logstash would leave the `host` field alone and use the client[3] ip as part of the `@source_host`.
- previous versions did not remove the leading `_` from keys. In order to smooth the upgrade process, make this optional
with new `strip_leading_underscore` (boolean) config attribute. It's useful to keep this separate from `remap` since this
 restores previous behavior.