stats: add keyed loop observer removal to fix use-after-free

[afrind]

QuicStatsCollector::Callback registered an EVB loop observer capturing
data_.get() as a raw pointer. If the Callback was destroyed while its
EventBaseStatsCollector was still alive, the observer would continue to
fire and dereference the freed pointer.

Fix by giving addLoopObserver a key (typically the owning object's this)
and adding removeLoopObserver(key), which posts an erase to the EVB thread.
Callback's destructor now calls removeLoopObserver so the subscription is
torn down safely. PicoQuicStatsCollector's observer is similarly keyed
for consistency.

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Stack created with Sapling. Best reviewed with ReviewStack.

• -> stats: add keyed loop observer removal to fix use-after-free #325
• relay: adopt new forwardChanged(forwarder, bool) callback signature #324
• relay: drop redundant setDownstream() from PendingTrackConsumer #323

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This change is 

[gmarzot]

@gmarzot reviewed 7 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on akash-a-n, michalhosna, mondain, Oxyd, peterchave, suhasHere, and TimEvens).