Item12104: Convert eachEventSince to a fixed layout

This should not impact existing applications.  The layout does not
change for "info" log levels, which is currently hard-coded in
Foswiki::Func.  Anyone using the logger->eachEventSince API directly for
log levels other than "info" will have to change.

This change returns log records in a fixed layout array.

(timestamp, user, action, web.topic, extra, remote-ip)

Levels other than info return (timestamp , , , extra, ) with missing
fields null, not undefined.

This change breaks the unit tests for LogDispatchContrib, which needs to
be updated.

More work to go.  eachEventSince needs to support returning multiple
levels.

git-svn-id: http://svn.foswiki.org/trunk@16422 0b4bb1d4-4e5a-0410-9cc4-b2b747904278