Added: Test to ensure remote ID stays the same for new draft
There may be multiple links between the same two objects
CS
Implemented: Create relation associated with field
Tested & Fixed: Loading field relations
Joined the 3 relation tables again

Cascade on delete does not work with this triple compund key. The
"optimization" does not really make sense anyways, since the foreign key
for field types also works with the null'ed column. Also types are just
integers, so we can also just inline them.

Makes the code a lot less complex.
Removed wrong / bullshit default statements
Throw incomplete exception if ID is not mapped
Added: Location mapping
Use existing mapped content ID as relation target
Fixed: Uniqueness of relations
Simplified code
Use ID Mapping in relation list tests
Unified on usage of "content" as type identifier
Migrated all tests to use "content" as identifier
Do not call generateId() for non-existing IDs
Do not map non-existing IDs

Also use consistently PHP_INT_MAX for non-existing IDs
Completed mapping from available data