faster parachain registration for polkadot-launch tests

[4meta5]

Opening this for discussion, not for review or to be merged.

There are two approaches to make parachain registration faster for the polkadot-launch tests:

1. register parachains in relay chain genesis
2. shorten the block time for the first two sessions

See #311 (comment) for an explanation of (1). I like it because it would make the ParaLifecycle of the parachains registered at genesis Parachain immediately. The way it works now, we register after genesis and then the ParaLifecycle is Onboarding for 2 sessions. During this time, we cannot open a channel with other parachains which is required for testing XCMP.

There is a genesis config for the paras pallet, but it is not currently exposed in the rococo runtime:

Paras: parachains_paras::{Pallet, Call, Storage},

So in order to pursue (1), we first need to add Config<T> there (PR polkadot).

[4meta5]

Should be sufficient for our testing purposes moonbeam-foundation/polkadot#1

but it's not building locally now :/

[4meta5]

Required change made in https://github.com/paritytech/polkadot/pull/2886/files , will close this for now and someone else can bring it back