core: optimize HasBlock check for recent blocks

When block is being spread through the network we can get a lot of invs with
the same hash. Some more stale nodes may also announce previous or some
earlier block. We can avoid full DB lookup for them and minimize inv handling
time (timeouts in inv handler had happened in #2744).

It doesn't affect tests, just makes node a little less likely to spend some
considerable amount of time in the inv handler.

pkg/core/blockchain.go

@@ -1714,6 +1714,16 @@ func (bc *Blockchain) HasTransaction(hash util.Uint256) bool {
 // HasBlock returns true if the blockchain contains the given
 // block hash.
 func (bc *Blockchain) HasBlock(hash util.Uint256) bool {
+	var height = bc.BlockHeight()
+	bc.headerHashesLock.RLock()
+	for i := int(height); i >= int(height)-4 && i >= 0; i-- {
+		if hash.Equals(bc.headerHashes[i]) {
+			bc.headerHashesLock.RUnlock()
+			return true
+		}
+	}
+	bc.headerHashesLock.RUnlock()
+
 	if header, err := bc.GetHeader(hash); err == nil {
 		return header.Index <= bc.BlockHeight()
 	}