API for task log and scheduled task (#25)

* API revision

* update

* make status a bitmap

* update api

* tests working

* new task log APIs

* update APIs

* write binary structures to redis

* update tests

* fix clang-format

* Fix formatting.

common.h

@@ -38,6 +38,8 @@
 
 typedef struct { unsigned char id[UNIQUE_ID_SIZE]; } unique_id;
 
+extern const unique_id NIL_ID;
+
 /* Generate a globally unique ID. */
 unique_id globally_unique_id(void);
 
@@ -46,11 +48,6 @@ unique_id globally_unique_id(void);
  * UNIQUE_ID_SIZE + 1 */
 char *sha1_to_hex(const unsigned char *sha1, char *buffer);
 
-/* Convert a hexdecimal string of length 40 to a 20 byte sha1 hash. This
- * function assumes that sha1 points to an already allocated char array of size
- * UNIQUE_ID_SIZE. */
-int hex_to_sha1(const char *hex, unsigned char *sha1);
-
 typedef unique_id object_id;
 
 #endif