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.
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