context UPDATE support for storing leafref references (#2155)

* Adding leafref references and API to lyd_node_term

* Adjusted to require context flag

* Extended the utest for tree_data

* Fixed typo in docs

* Refactored based on PR discussion

* Refactored to use hash table for leafref nodes
diff --git a/src/common.h b/src/common.h
index 0fedeae..0157d26 100644
--- a/src/common.h
+++ b/src/common.h
@@ -373,6 +373,7 @@
     void *ext_clb_data;               /**< optional private data for ::ly_ctx.ext_clb */
     struct ly_ht *err_ht;             /**< hash table of thread-specific list of errors related to the context */
     pthread_mutex_t lyb_hash_lock;    /**< lock for storing LYB schema hashes in schema nodes */
+    struct ly_ht *leafref_links_ht;   /**< hash table of leafref links between term data nodes */
 };
 
 /**