Preserve consistency when removing all nodes from a hash table

Preserve consistency when removing all nodes from a hash table

During the recent refactorings of GHashTable a bug was introduced
where removing all nodes from a hash table would leave tombstones
behind, but make the counts appear like there are none.

Reported and tracked down by Carlos Garnacho,
https://bugzilla.gnome.org/show_bug.cgi?id=651141

This commit also adds a test that checks the internal consistency
of GHashTable over several insert/remove/remove-all operations.