Staking/session: Rename the interactions

[kianenigma]

The naming of the interactions between session and staking is a bit confusing. Here's a proposal from a different discussion with @thiolliere:

new_session -> schedule_session
start_session -> activate_session
current_era -> scheduled_era
active_era -> active_era
ErasStartSessionIndex -> ActiveErasStartSessionIndex

Will sadly need a runtime migration though.

[gui1117]

current_era -> scheduled_era

maybe better latest_scheduled_era. because there can be multiple era scheduled ahead of the active_era.

ErasStartSessionIndex -> ScheduledErasStartSessionIndex

Note that currently this storage holds the information for all eras in latest_scheduled_era - HISTORY_DEPTH to latest_scheduled_era.
Thus it also contains the information active_era and below. Though I think we don't use it so maybe we could clear this storage when era is activated.

[gui1117]

We can also rename EstimateNextNewSession to EstimateNextScheduleSession

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.

[gui1117]

I still think we should do that

[michalisFr]

Since current_era (or scheduled_era) affects many aspects of staking, including when the unbonding period ends, can we add the start timestamp in chain state, similarly to active_era?