Drop gvdb_table_new_from_data() and add gvdb_table_new_from_bytes().
Since the underlying backingstore of a GvdbTable is now always
refcounted, drop the refcounting on GvdbTable itself.
The attempt at the simple method for preventing unbounded recursion
proved to be insufficient due to the existence of dconf databases in the
wild that violated the rule (leading to the entire content of the
database being scrapped). It also still had the ugly assert for less
than 64 levels of recursion that could have been hit by a determined
advisary.
gvdb_table_get_names() allows the dconf-service to do everything it
needs without the troubles associated with the walk approach.
