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/include/types/backend.h b/include/types/backend.h
index 6325be2..446ac2a 100644
--- a/include/types/backend.h
+++ b/include/types/backend.h
@@ -115,6 +115,7 @@
 #define BE_LB_HFCN_SDBM   0x000000 /* sdbm hash */
 #define BE_LB_HFCN_DJB2   0x400000 /* djb2 hash */
 #define BE_LB_HFCN_WT6    0x800000 /* wt6 hash */
+#define BE_LB_HFCN_CRC32  0xC00000 /* crc32 hash */
 #define BE_LB_HASH_FUNC   0xC00000 /* get/clear hash function */