Recopy key instead of steal when undeclaring

[jean-roland]

This fixes #282, the undeclare process for queryable and subscription was:

1) steal the keyexpr
2) send the declare N message
3) remove subscription/queryable from session

The delay between 1) and 3) was the source of the problem and stealing the keyepr was probably useful before the introduction of refcounting for subscriptions and queryables, but it is no longer necessary.