safe multi-era slashing for NPoS (#3846)

* define slashing spans

* tests and pruning for slashing-spans record

* validators get slashed before nominators

* apply slash to nominators as well

* chill and end slashing spans

* actually perform slashes

* integration (tests failing)

* prune metadata

* fix compilation

* some tests for slashing and metadata garbage collection

* correctly pass session index to slash handler

* test span-max property for nominators and validators

* test that slashes are summed correctly

* reward value computation

* implement rewarding

* add comment about rewards

* do not adjust slash fraction in offences module

* fix offences tests

* remove unused new_offenders field

* update runtime version

* fix up some docs

* fix some CI failures

* remove no-std incompatible vec! invocation

* try to fix span-max rounding error

* Update srml/staking/src/slashing.rs

Fix type: winow -> window

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* slashes from prior spans don't kick validator again

* more information for nominators, suppression

* ensure ledger is consistent with itself post-slash

* implement slash out of unlocking funds also

* slashing: create records to be applied after-the-fact

* queue slashes for a few eras later

* method for canceling deferred slashes

* attempt to fix test in CI

* storage migration for `Nominators`

* update node-runtime to use SlashDeferDuration

* adjust migration entry-points somewhat

* fix migration compilation

* add manual Vec import to migration

* enable migrations feature in node-runtime

* bump runtime version

* update to latest master crate renames

* update to use ensure-origin

* Apply suggestions from code review

use `ensure!`

Co-Authored-By: Gavin Wood <gavin@parity.io>

* fix multi-slash removal

* initialize storage version to current in genesis

* add test for version initialization

substrate/frame/offences/src/mock.rs

@@ -46,6 +46,7 @@ impl<Reporter, Offender> offence::OnOffenceHandler<Reporter, Offender> for OnOff
 	fn on_offence(
 		_offenders: &[OffenceDetails<Reporter, Offender>],
 		slash_fraction: &[Perbill],
+		_offence_session: SessionIndex,
 	) {
 		ON_OFFENCE_PERBILL.with(|f| {
 			*f.borrow_mut() = slash_fraction.to_vec();
@@ -148,9 +149,7 @@ impl<T: Clone> offence::Offence<T> for Offence<T> {
 	}
 
 	fn session_index(&self) -> SessionIndex {
-		// session index is not used by the pallet-offences directly, but rather it exists only for
-		// filtering historical reports.
-		unimplemented!()
+		1
 	}
 
 	fn slash_fraction(