Overwinter peer management and network handshaking.

Implements ZIP 201.

src/net.h

@@ -57,6 +57,8 @@ static const size_t MAPASKFOR_MAX_SZ = MAX_INV_SZ;
 static const size_t SETASKFOR_MAX_SZ = 2 * MAX_INV_SZ;
 /** The maximum number of peer connections to maintain. */
 static const unsigned int DEFAULT_MAX_PEER_CONNECTIONS = 125;
+/** The period before a network upgrade activates, where connections to upgrading peers are preferred (in blocks). */
+static const int NETWORK_UPGRADE_PEER_PREFERENCE_BLOCK_PERIOD = 24 * 24 * 3;
 
 unsigned int ReceiveFloodSize();
 unsigned int SendBufferSize();