MEDIUM: backend: add the crc32 hash algorithm for load balancing

Since we have it available, let's make it usable for load balancing,
it comes at no cost except 3 lines of documentation.
diff --git a/src/backend.c b/src/backend.c
index 35edf75..721c12a 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -75,6 +75,9 @@
 	case BE_LB_HFCN_WT6:
 		hash = hash_wt6(key, len);
 		break;
+	case BE_LB_HFCN_CRC32:
+		hash = hash_crc32(key, len);
+		break;
 	case BE_LB_HFCN_SDBM:
 		/* this is the default hash function */
 	default: