Make a collator send a collation as backup as well (#1353)
Currently a collator will only send a collation to validators it is a primary for. While testing this could lead to the situation that the same collator was registered as prime for all Parachain validators but failed for other reasons to generate a PoVBlock. However no other collator was sending a collation, which stopped the Parachain until the faulty collator was stopped. This pr solves this problem by making sure that every collator sends a collation to one of his validators he is connected to, but registered as backup.