Make event specs pass under event_v1

This required implementing Event#append. Some semantics have changed.
"@message" and "@tags" are no longer handled specially (mainly because
they aren't described by event v1 schema). This likely has broken the
multiline filter tests (intentionally). The multiline filter will need
to have custom merging of certain fields (like joining message with
newlines instead of putting it all in an array)
- all event tests now passing with event v1
- make event v1 the default.