Persist evidence in equivocation handler (#8502)

* Persist evidence in handler

* update docs

* update docs

Co-authored-by: Alessio Treglia <alessio@tendermint.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -48,6 +48,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 * (x/{bank,distrib,gov,slashing,staking}) [\#8363](https://github.com/cosmos/cosmos-sdk/issues/8363) Store keys have been modified to allow for variable-length addresses.
 * (x/ibc) [\#8266](https://github.com/cosmos/cosmos-sdk/issues/8266) Add amino JSON for IBC messages in order to support Ledger text signing.
+* (x/evidence) [\#8502](https://github.com/cosmos/cosmos-sdk/pull/8502) `HandleEquivocationEvidence` persists the evidence to state.
 
 ### Improvements
 

x/evidence/keeper/infraction.go

@@ -119,4 +119,5 @@ func (k Keeper) HandleEquivocationEvidence(ctx sdk.Context, evidence *types.Equi
 
 	k.slashingKeeper.JailUntil(ctx, consAddr, types.DoubleSignJailEndTime)
 	k.slashingKeeper.Tombstone(ctx, consAddr)
+	k.SetEvidence(ctx, evidence)
 }

x/evidence/keeper/infraction_test.go

@@ -69,6 +69,10 @@ func (suite *KeeperTestSuite) TestHandleDoubleSign() {
 	tstaking.Ctx = ctx
 	tstaking.Denom = stakingParams.BondDenom
 	tstaking.Undelegate(sdk.AccAddress(operatorAddr), operatorAddr, totalBond, true)
+
+	// query evidence from store
+	evidences := suite.app.EvidenceKeeper.GetAllEvidence(ctx)
+	suite.Len(evidences, 1)
 }
 
 func (suite *KeeperTestSuite) TestHandleDoubleSign_TooOld() {

x/evidence/spec/01_concepts.md

@@ -66,7 +66,8 @@ The `Handler` (defined below) is responsible for executing the entirety of the
 business logic for handling `Evidence`. This typically includes validating the
 evidence, both stateless checks via `ValidateBasic` and stateful checks via any
 keepers provided to the `Handler`. In addition, the `Handler` may also perform
-capabilities such as slashing and jailing a validator.
+capabilities such as slashing and jailing a validator. All `Evidence` handled 
+by the `Handler` should be persisted.
 
 ```go
 // Handler defines an agnostic Evidence handler. The handler is responsible