Add a getutxos command to the p2p protocol. It allows querying of the UTXO set

given a set of outpoints.

src/protocol.h

@@ -64,6 +64,7 @@ class CMessageHeader
 enum
 {
     NODE_NETWORK = (1 << 0),
+    NODE_GETUTXOS = (1 << 1),
 
     // Bits 24-31 are reserved for temporary experiments. Just pick a bit that
     // isn't getting used, or one not being used much, and notify the