invbloom: reduce hash count to 3.

Kalle shows the superiority of 3 (as does the paper for > 50 buckets).

https://github.com/kallerosenbaum/bitcoin-iblt/wiki/Diff-count-VS-failure-probabilitySigned-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/invbloom/invbloom.c

@@ -10,7 +10,7 @@
 
 	Eppstein, David, et al. "What's the difference?: efficient set reconciliation without prior context." ACM SIGCOMM Computer Communication Review. Vol. 41. No. 4. ACM, 2011. http://conferences.sigcomm.org/sigcomm/2011/papers/sigcomm/p218.pdf
 */
-#define NUM_HASHES 4
+#define NUM_HASHES 3
 
 struct invbloom *invbloom_new_(const tal_t *ctx,
 			       size_t id_size,