Shorten transaction scheduler leader

[pro-wh]

From #1640, the transaction scheduler committee has a single leader for an entire general purpose epoch. We need it to rotate on a shorter interval.

Details

Two current proposals:

1. Change leader among the committee on a new "sub-epoch" system
2. Let different kinds of committees have different epoch times, with the transaction scheduler committee having a shorter epoch time (related to storage committees too Storage nodes should register with the registry #1585 )

Clients will have to be aware of this too.

blocks #1577

Acceptance Criteria

• we can shorten how long a node will be transaction scheduler leader without shortening the compute committee lifetime

[Yawning]

I suspect we will need both eventually, but the alterations for #2 are likely to be easier.

[ptrus]

do we want to do this before / after (or together with) #642 (i assume this is the planned rewrite)? I guess it's independent if we plan to keep the trivial scheduler? (not sure if this is true)

[pro-wh]

Shouldn't #642 be done already? We already have a tendermint backed committee scheduler. As for @kostko 's comment on #642, @ptrus you've already implemented that in the node role system.

[kostko]

We do not have a Tendermint backed committee scheduler, there is only the trivial implementation which is not on chain at all (e.g., it is not implemented as part of our ABCI app).

[pro-wh]

but it is entirely computed from on-chain information and driven by chain progression

[pro-wh]

Decoupling the intervals of different systems' committees would help us test committee change logic independently

cc #1669 (comment)