Fetch multiple pages in parallel with full option in findAll

[edoardocavazza]

Calling the findAll Collection's method with the option full: true should fetch necessary data with parallel requests and then join the collections using the correct order.

At the moment, this is impossibile because multiple RelationshipCollection instances use the same internal array instance, so the order of added items depends on the resolution of the fetch call.

This PR temporarily fixes the joinCollection behavior for RelationshipCollection which causes duplicated entries in the collection.

To do:

• Allow multiple RelationshipCollection instances for the same model
• Exec multiple pages fetch in parallel

[fquffio]

Some initial work on parallel fetching of multiple pages has been done in f92fc46, then reverted.