require block.Time of the fist block to be genesis time

Refs #2587:

```
We only start validating block.Time when Height > 1, because there is no
commit to compute the median timestamp from for the first block. This
means a faulty proposer could make the first block with whatever time
they want.

Instead, we should require the timestamp of block 1 to match the genesis
time.

I discovered this while refactoring the ValidateBlock tests to be
table-driven while working on tests for #2560.
```

state/state.go

@@ -118,10 +118,7 @@ func (state State) MakeBlock(
 
 	// Set time
 	if height == 1 {
-		block.Time = tmtime.Now()
-		if block.Time.Before(state.LastBlockTime) {
-			block.Time = state.LastBlockTime // state.LastBlockTime for height == 1 is genesis time
-		}
+		block.Time = state.LastBlockTime // genesis time
 	} else {
 		block.Time = MedianTime(commit, state.LastValidators)
 	}

state/validation.go

@@ -123,6 +123,15 @@ func validateBlock(stateDB dbm.DB, state State, block *types.Block) error {
 				block.Time,
 			)
 		}
+	} else if block.Height == 1 {
+		genesisTime := state.LastBlockTime
+		if !block.Time.Equal(genesisTime) {
+			return fmt.Errorf(
+				"Block time %v is not equal to genesis time %v",
+				block.Time,
+				genesisTime,
+			)
+		}
 	}
 
 	// Limit the amount of evidence

state/validation_test.go

@@ -2,6 +2,7 @@ package state
 
 import (
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 	"github.com/tendermint/tendermint/crypto/ed25519"
@@ -32,11 +33,11 @@ func TestValidateBlockHeader(t *testing.T) {
 		name          string
 		malleateBlock func(block *types.Block)
 	}{
-		{"ChainID wrong", func(block *types.Block) { block.ChainID = "not-the-real-one" }}, // wrong chain id
-		{"Height wrong", func(block *types.Block) { block.Height += 10 }},                  // wrong height
-		// TODO(#2589) (#2587) : {"Time", func(block *types.Block) { block.Time.Add(-time.Second * 3600 * 24) }}, // wrong time
-		{"NumTxs wrong", func(block *types.Block) { block.NumTxs += 10 }},     // wrong num txs
-		{"TotalTxs wrong", func(block *types.Block) { block.TotalTxs += 10 }}, // wrong total txs
+		{"ChainID wrong", func(block *types.Block) { block.ChainID = "not-the-real-one" }},
+		{"Height wrong", func(block *types.Block) { block.Height += 10 }},
+		{"Time wrong", func(block *types.Block) { block.Time = block.Time.Add(-time.Second * 3600 * 24) }},
+		{"NumTxs wrong", func(block *types.Block) { block.NumTxs += 10 }},
+		{"TotalTxs wrong", func(block *types.Block) { block.TotalTxs += 10 }},
 
 		{"LastBlockID wrong", func(block *types.Block) { block.LastBlockID.PartsHeader.Total += 10 }},
 		{"LastCommitHash wrong", func(block *types.Block) { block.LastCommitHash = wrongHash }},