Support disabling validators.

[dvc94ch]

Instead of forcing a new era when the staking module slashes a validator it disables the validator. The default implementation for this forces a new era, but the babe module should be able to do it's own book keeping.

[gavofyork]

This isn't quite right. Is it only meant as the first step with several follow-up PRs yet to come?

Staking shouldn't be implementing or responding to OnDisable directly at all. The only signal Staking should be responding to is SessionWillEnd. The only modules that should be listening are Grandpa, Babe and (in Polkadot) Parachains. Session should have a configurable type in its trait to tell it when the session is_ended, which for now can be a simple block_number() % SESSION_LENGTH == 0, but will ultimately come from Babe.

[dvc94ch]

Is SessionWillEnd distinct from OnSessionChange?