Current if index_node throws when trying to
add an already indexed node, pop_node might
unindex the existing node instead of the new one.
Instead, with this change, unindex_node looks up
the node by its pointer and removed it from the
index map only if it's found there so to clean up
safely after index_node throws (at any stage).
Add a unit test to verify that.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
e29994b2aa