Attempt to integrate nimbus without keystore support

[JoshOrndorff]

What does it do?

Updates cumulus dependencies to latest rococo-v1 branches, and strives to bring in the nimbus consensus framework (as opposed to the original PoC crates that we sketched here). Currently the PoC crates are still in use.

Target dependencies are:

Substrate:
source = "git+https://github.com/paritytech/substrate?branch=rococo-v1#2be8fcc4236d32786c62f6f27a98e7fe7e550807"

Polkadot:
source = "git+https://github.com/paritytech/polkadot?branch=rococo-v1#127eb17a25bbe2a9f2731ff11a65d7f8170f2373"

Cumulus:
source = "git+https://github.com/purestake/cumulus?branch=nimbus#48b8bd898913c9f64ea754d6368290075aaef97a"

Frontier:
source = "git+https://github.com/purestake/frontier?branch=joshy-substrate-2be8fcc4#b32e61460e1ecc555df24566fdb961f6e3d05b7c"

What important points reviewers should know?

This is using a fork of cumulus (my nimbus branch). It remains to be seen whether nimbus will be accepted into cumulus, or move to its own repo, but either way the long-term plan is not to maintain a fork.

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

Basti's ongoing effort to wire sc-consensus-aura into the parachain context paritytech/cumulus#371

Checklist

• ✔️ Does it require a purge of the network?
• You bumped the runtime version if there are breaking changes in the runtime ?
• ✔️ Does it require changes in documentation/tutorials ?

[JoshOrndorff]

This PR got derailed into an attempt to add part of nimbus without adding all of it. Other bugs were fixed along the way, so I'll close this for now. Let's keep the branch just in case this is ever useful. It may be that this is a valid usecase for nimbus in which case I'll need to move som code from this branch back over there.