Fixed a leak in ShieldedSeq.

I don't understand why, but if the discarded ItemKeepers' Nexts kept pointing
at other items (last one eventually pointing at current queue contents) they would
not be collected, and memory would grow... Running on Mono 2.10.8.1. Setting the
Next to null before discarding the item changes things dramatically, and 1 million items
is pushed through the queue with mem consumption stable for the 50 seconds it takes.