Cumulus: Allow aura to use initialized collation request receiver (#1911)

When launching our [small
network](https://github.com/paritytech/polkadot-sdk/blob/master/cumulus/zombienet/examples/small_network.toml)
for testing the node was crashing here shortly after launch:

https://github.com/paritytech/polkadot-sdk/blob/5cdd819e/polkadot/node/collation-generation/src/lib.rs#L140

After changes in #1788 for the asset hub collator we are waiting for
blocks of the shell runtime to pass before we initialize aura. However,
this means that we attempted to initialize the collation related relay
chain subsystems twice, leading to the error.

I modified Aura to let it optionally take an already initialized stream
of collation requests.