core: don't always store all hashes in memory

We're paging these hashes, so we need a previous full page and a current one
plus some cache for various requests. Storing 1M of hashes is 32M of memory
and it grows quickly. It also seriously affects node startup time, most of
what it's doing is reading these hashes, the longer the chain the more time it
needs to do that.

Notice that this doesn't change the underlying DB scheme in any way.

pkg/core/blockchain_core_test.go

@@ -225,7 +225,7 @@ func TestBlockchain_InitWithIncompleteStateJump(t *testing.T) {
 	t.Run("invalid state sync point", func(t *testing.T) {
 		bcSpout.dao.Store.Put(bPrefix, []byte{byte(stateJumpStarted)})
 		point := make([]byte, 4)
-		binary.LittleEndian.PutUint32(point, uint32(len(bcSpout.headerHashes)))
+		binary.LittleEndian.PutUint32(point, bcSpout.lastHeaderIndex()+1)
 		bcSpout.dao.Store.Put([]byte{byte(storage.SYSStateSyncPoint)}, point)
 		checkNewBlockchainErr(t, boltCfg, bcSpout.dao.Store, "invalid state sync point")
 	})