evidence: cap evidence to an absolute number (#4780)

The number of evidence that can be committed in a single block is capped by a new evidence parameter called MaxNum

abci/types/types.pb.go

@@ -2644,6 +2644,7 @@ type EvidenceParams struct {
 	// Note: must be greater than 0
 	MaxAgeNumBlocks      int64         `protobuf:"varint,1,opt,name=max_age_num_blocks,json=maxAgeNumBlocks,proto3" json:"max_age_num_blocks,omitempty"`
 	MaxAgeDuration       time.Duration `protobuf:"bytes,2,opt,name=max_age_duration,json=maxAgeDuration,proto3,stdduration" json:"max_age_duration"`
+	MaxNumEvidence       uint32        `protobuf:"varint,1,opt,name=max_num_evidence,json=maxNumEvidence,proto3" json:"max_num_evidence,omitempty"`
 	XXX_NoUnkeyedLiteral struct{}      `json:"-"`
 	XXX_unrecognized     []byte        `json:"-"`
 	XXX_sizecache        int32         `json:"-"`

abci/types/types.proto

@@ -302,6 +302,7 @@ message EvidenceParams {
   int64                    max_age_num_blocks = 1;
   google.protobuf.Duration max_age_duration   = 2
     [(gogoproto.nullable) = false, (gogoproto.stdduration) = true];
+  uint32                   max_num            = 3;
 }
 
 // ValidatorParams contains limits on validators.