Add receiver_changes to track changes to an object

[lleger]

We should add another JSONB fields called receiver_changes to track the actual changes to the object. I think we could use ActiveModel::Dirty to accomplish this.

There's two areas of difficulty with this:

1. The changes get wiped once #save is called, so we may have to use #previous_changes
2. ActiveJob won't serialize a timestamp, so we may have to sanitize that array (perhaps we can add a proc to the config for that)

Here's some README language I wrote before I attempted to add this earlier:

### Metadata

Transcript tracks some metadata about the audit entry. It logs the receiver at the top of the entry to `receiver_serialized`. This is a JSONB column in PostgreSQL, which can be indexed and queried.

The changes to the receiver, via [`ActiveModel::Dirty`](http://api.rubyonrails.org/classes/ActiveModel/Dirty.html) are also recorded in `receiver_changes`. This is also a JSONB column.

This metadata should allow you to reconstruct the object in either the before or after state.