BIP144: Handshake and relay (receiver side)

Service bit logic by Nicolas Dorier.

Only download blocks from witness peers after fork.

src/protocol.h

@@ -264,6 +264,9 @@ enum ServiceFlags : uint64_t {
     // Bitcoin Core nodes used to support this by default, without advertising this bit,
     // but no longer do as of protocol version 70011 (= NO_BLOOM_VERSION)
     NODE_BLOOM = (1 << 2),
+    // Indicates that a node can be asked for blocks and transactions including
+    // witness data.
+    NODE_WITNESS = (1 << 3),
 
     // 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