Amend flamsteed serialisation to ensure event uniqueness.

Individual events are currently flattened and merged together during serialization. 

We want to avoid this to be able to process and persist each event individually, but reference the parent page impression.

This PR:

* Maintains each event as separate entities during serialization
* Includes `t` `session_id` `fid` and `schema` attributes on each log entry
* Increases protection against unnecessary data pushes via image pixel
* Improves efficiency of serialization using current index, rather than `Array#length`