Strange space leak in PBFT.ChainState

It has been reported tthat https://github.com/input-output-hk/ouroboros-network/commit/f2a050ba9ada3bf3ee2421f5e610947619d28337 introduced a space leak. If we remove the following line, the space leak is gone: https://github.com/input-output-hk/ouroboros-network/blob/f2a050ba9ada3bf3ee2421f5e610947619d28337/ouroboros-consensus/src/Ouroboros/Consensus/Protocol/PBFT/ChainState.hs#L356
Note that the space leak has _nothing_ to do with EBBs themselves! Even with an empty `ebbs`, we still have the space leak.

Looking at the implementation of `pruneEBBsLT`: https://github.com/input-output-hk/ouroboros-network/blob/f2a050ba9ada3bf3ee2421f5e610947619d28337/ouroboros-consensus/src/Ouroboros/Consensus/Protocol/PBFT/ChainState.hs#L624
No matter how I try to force `anchorSlot cs` (bangs, `seq`, pass it in as an argument, ...), it causes a thunk in the `StrictSeq` fields. Even if I pattern match on it, ignore it (!) and do `Map.map id` instead of filtering. Strangely, `cs { ebbs = ebbs }` doesn't have the leak.

If we add `{-# OPTIONS_GHC -O0 #-}` to the module, the leak goes away. The leak is there with `-O1` and `-O2` (`-fno-full-laziness` doesn't help). This makes me think the leak is caused by a bug in GHC :scream:.

You can use the following snippet to quickly detect the leak:
```haskell
main :: IO ()
main = do
varCS <- Strict.newTVarWithInvariantM unsafeNoThunks CS.empty
let go slot@(SlotNo s) = do
when (s `mod` 1000 == 0) $ print s
atomically $ modifyTVar varCS $
CS.append securityParam windowSize (signerForSlot slot)
go (succ slot)
go 0
where
securityParam = SecurityParam 2
windowSize = CS.WindowSize 2
signerForSlot :: SlotNo -> CS.PBftSigner PBftMockCrypto
signerForSlot slot@(SlotNo s) = CS.PBftSigner
{ pbftSignerSlotNo = slot
, pbftSignerGenesisKey = Crypto.VerKeyMockDSIGN (fromIntegral s `mod` 7)
}
```
The thunk detection (`unsafeNoThunks`) will catch the thunk. Profiling the heap confirms that thunk detection is correct.