Investigate epoch descriptor

[drskalman]

Investigate the role of the epoch descriptor defined here:

https://github.com/paritytech/substrate/blob/master/client/consensus/epochs/src/lib.rs#L212

And spec if it is necessary for the spec to contains it.

[FlorianFranzen]

According to the current host spec and the BABE research write up, the slot number is supposed to be reset at the beginning of each epoch.

Substrate does not do this, so it needs epoch descriptors to tracks the slot number at the beginning (and end) of each epoch. Therefore there is no need to add them to the spec at the moment.

[drskalman]

the original description for the developer is:
"A new post-digest type NextConfigDescriptor is added. This descriptor can only be used at epoch transition, on the same block as NextEpochDescriptor. NextConfigDescriptor is optional, and modifies the config value of c and allowed_slots of the next upcoming epoch (same as the block NextEpochDescriptor points to)." in Issue #46

[FlorianFranzen]

Indeed I think we closed this too soon. While we might not have to spec the ViableEpochDescriptor, we should take another look at the NextEpochDescriptor.

[FlorianFranzen]

This is included in #168 now.