rpc: Improve getblockstats

- Fix getblockstats for block height 0 which previously returned an error.
- Introduce alternative utxo_*_actual statistics which exclude unspendables: Genesis block, BIP30, unspendable outputs
- Update test data
- Explicitly test Genesis block results

test/functional/data/rpc_getblockstats.json

@@ -142,7 +142,9 @@
       "totalfee": 0,
       "txs": 1,
       "utxo_increase": 2,
-      "utxo_size_inc": 163
+      "utxo_increase_actual": 1,
+      "utxo_size_inc": 163,
+      "utxo_size_inc_actual": 75
     },
     {
       "avgfee": 4460,
@@ -179,7 +181,9 @@
       "totalfee": 4460,
       "txs": 2,
       "utxo_increase": 3,
-      "utxo_size_inc": 236
+      "utxo_increase_actual": 2,
+      "utxo_size_inc": 236,
+      "utxo_size_inc_actual": 148
     },
     {
       "avgfee": 24906,
@@ -216,7 +220,9 @@
       "totalfee": 74720,
       "txs": 4,
       "utxo_increase": 5,
-      "utxo_size_inc": 384
+      "utxo_size_inc": 384,
+      "utxo_increase_actual": 4,
+      "utxo_size_inc_actual": 296
     }
   ]
-}
+}