cryptonote_protocol_handler: prevent potential DoS

Essentially, one can send such a large amount of IDs that core exhausts
all free memory. This issue can theoretically be exploited using very
large CN blockchains, such as Monero.

This is a partial fix. Thanks and credit given to CryptoNote author
'cryptozoidberg' for collaboration and the fix. Also thanks to
'moneromooo'. Referencing HackerOne report #506595.

src/cryptonote_protocol/cryptonote_protocol_handler.h

@@ -52,6 +52,7 @@ PUSH_WARNINGS
 DISABLE_VS_WARNINGS(4355)
 
 #define LOCALHOST_INT 2130706433
+#define CURRENCY_PROTOCOL_MAX_BLOCKS_REQUEST_COUNT 500
 
 namespace cryptonote
 {