Set TCP_NODELAY on P2P sockets.

Nagle appears to be a significant contributor to latency now that the static
 sleeps are gone.  Most of our messages are relatively large compared to
 IP + TCP so I do not expect this to create enormous overhead.

This may also reduce traffic burstyness somewhat.

src/compat.h

@@ -38,6 +38,7 @@
 #include <sys/types.h>
 #include <net/if.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 #include <arpa/inet.h>
 #include <ifaddrs.h>
 #include <limits.h>