modules/kvs: add kvs_getat()

Add a new function kvs_getat() which is like kvs_get() but has
a new "treeobj" argument that specifies the specific root
(or other directory) "snapshot" that the key will be looked
up in.

Partially fixes flux-framework#64

src/modules/kvs/kvs.h

@@ -47,6 +47,11 @@ int kvs_get_symlink (flux_t h, const char *key, char **valp);
  */
 int kvs_get_treeobj (flux_t h, const char *key, char **treeobj);
 
+/* Like kvs_get() but lookup is relative to 'treeobj'.
+ */
+int kvs_getat (flux_t h, const char *treeobj,
+               const char *key, char **json_str);
+
 /* kvs_watch* is like kvs_get* except the registered callback is called
  * to set the value.  It will be called immediately to set the initial
  * value and again each time the value changes.

src/modules/kvs/libkvs.c

@@ -380,6 +380,29 @@ int kvs_get (flux_t h, const char *key, char **val)
     return 0;
 }
 
+int kvs_getat (flux_t h, const char *treeobj,
+               const char *key, char **val)
+{
+    JSON v = NULL;
+    JSON dirent = NULL;
+
+    if (!treeobj || !key || !(dirent = Jfromstr (treeobj))
+                         || dirent_validate (dirent) < 0) {
+        errno = EINVAL;
+        goto error;
+    }
+    if (getobj (h, dirent, key, 0, &v) < 0)
+        goto error;
+    if (val)
+        *val = xstrdup (Jtostr (v));
+    Jput (dirent);
+    return 0;
+error:
+    Jput (v);
+    Jput (dirent);
+    return -1;
+}
+
 /* deprecated */
 int kvs_get_obj (flux_t h, const char *key, JSON *val)
 {