MB-16181: Build SystemEvent keys with the collections separator

MB-16181: Build SystemEvent keys with the collections separator

The keys were fixed as $collections::<event> but are now changed
so that the :: is the collections separator.

This allows code to split the event key if they wish using the
same code they would split document keys.

Change-Id: I48575d295f8c058a79cf208fe3c9d3a9b3c9ed15
Reviewed-on: http://review.couchbase.org/78136
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>