Alternate message field support without clobbering
Alternate message field support without clobbering

The proposed branch for log/record/message would write over (clobber)
short_message, or any previously-copied alternate message field, causing field
loss if more than one of these fields was set.

Instead, move this logic to short_message validation, with a simple priority
match of various alternate field names/keys.

This also includes a restriction so that only non-empty fields can masquerade
as the short_message.

emsearcy/fluent-plugin-gelf,bodhi-space/fluent-plugin-gelf