core/state: merge spent and unspent coins state, use it to store more things

This change reduces pressure on DB by doing the following things:
 * not storing additional KV pair for SpentCoin
 * storing Output right in the UnspentCoin, thus eliminating the need to get a
   full transaction from DB

At the same time it makes UnspentCoin more fat and hot, but it should probably
worth it.

Also drop `GetUnspentCoinStateOrNew` as it shouldn't ever existed, UTXOs
can't come out of nowhere.

1.5M block import time (VerifyBlocks disabled) on AMD Ryzen 5 1600/16GB/HDD,
before:
real    302m9.895s
user    96m17.200s
sys     13m37.084s

after:
real    159m16.551s
user    69m58.279s
sys     7m34.334s

So it's almost two-fold which is a great improvement.

pkg/core/state/unspent_coin_test.go

@@ -3,18 +3,44 @@ package state
 import (
 	"testing"
 
+	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
+	"github.com/nspcc-dev/neo-go/pkg/internal/random"
 	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/nspcc-dev/neo-go/pkg/util"
 	"github.com/stretchr/testify/assert"
 )
 
 func TestDecodeEncodeUnspentCoin(t *testing.T) {
 	unspent := &UnspentCoin{
-		States: []Coin{
-			CoinConfirmed,
-			CoinSpent,
-			CoinSpent,
-			CoinSpent,
-			CoinConfirmed,
+		Height: 100500,
+		States: []OutputState{
+			{
+				Output: transaction.Output{
+					AssetID:    random.Uint256(),
+					Amount:     util.Fixed8(42),
+					ScriptHash: random.Uint160(),
+				},
+				SpendHeight: 201000,
+				State:       CoinSpent,
+			},
+			{
+				Output: transaction.Output{
+					AssetID:    random.Uint256(),
+					Amount:     util.Fixed8(420),
+					ScriptHash: random.Uint160(),
+				},
+				SpendHeight: 0,
+				State:       CoinConfirmed,
+			},
+			{
+				Output: transaction.Output{
+					AssetID:    random.Uint256(),
+					Amount:     util.Fixed8(4200),
+					ScriptHash: random.Uint160(),
+				},
+				SpendHeight: 111000,
+				State:       CoinSpent & CoinClaimed,
+			},
 		},
 	}