core: Move ssb.swapWithServerIdentity() to C.

src/ssb.db.h

@@ -647,4 +647,14 @@ bool tf_ssb_db_is_blocked(sqlite3* db, const char* id);
 */
 void tf_ssb_db_get_blocks(sqlite3* db, void (*callback)(const char* id, double timestamp, void* user_data), void* user_data);
 
+/**
+** Swap a user's identity with the server identity.
+** @param db The database.
+** @param user The user.
+** @param user_id The user identity.
+** @param server_id The server identity.
+** @return Null on success or an error message on error.  Free with tf_free().
+*/
+char* tf_ssb_db_swap_with_server_identity(sqlite3* db, const char* user, const char* user_id, const char* server_id);
+
 /** @} */