Revert Base#eql to == .

== is used to compare aliquot and sample are the same.
To speed it up, implement attributes.
Purge invalid object.

Delete invalid association.
Delete children (to_delete) properly.
Fix persistor dup not copying the right instance variables.
Add serialize method to Session ...

Use Marshall methods but for some reason it doesn't work with Sequel.
We might use Oj instead but that will need a migration to change the
dump format of the objects already in the database.
filter_on_load uses block.
add Persistor#qualified_key
Fix UuidResource saving state with no resource.
Misc Session.