Define authority directory for each epoch

[drskalman]

We need to spec how to retrieve the authority list at begining of each epoch
This is basically should be contained an algorithm which need to be called
at the begining of each epoch for babe. This also need to include how
to retrieve session key of each authority and how to verify them.

[FlorianFranzen]

This closely relates to specifying NextEpochDescriptor` #173.

[FlorianFranzen]

This also need to include how to retrieve session key of each authority and how to verify them.

This is the only thing really missing. @drskalman If you add a bit more details of what you think is missing, I can add this as well.

[drskalman]

Currently spec suggests how to update the authorities but if we have a central one algorithm that take care of all setting in and out of authorities, it is very helpful for the implementers.

[drskalman]

This needs to be waited till pull request #168 to be merged first.

[FlorianFranzen]

@lamafab Will track the remaining work in #121.