LedgerDB.V2: take snapshot for immutable tip

It is also sound to create a snapshot for the anchor (which might be a bit older
than the immutable tip, as garbage collection has not yet been performed); eg in
V1, we create a snapshot for the last flushed state, which is almost always
older than the immutable tip.

However, it is trivial to create a snapshot for the immutable tip, so let's just
do that, as it means (very slightly) faster replay on node startup.

Author: amesgen

ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Storage/LedgerDB/V2.hs

@@ -380,6 +380,7 @@ implTryTakeSnapshot bss env mTime nrBlocks =
           (LedgerDBSnapshotEvent >$< ldbTracer env)
           (ldbHasFS env)
         . anchorHandle
+        . volatileSuffix k
         =<< readTVarIO (ldbSeq env)
       Monad.void $
         trimSnapshots
@@ -390,6 +391,8 @@ implTryTakeSnapshot bss env mTime nrBlocks =
     else
       pure $ SnapCounters (fst <$> mTime) nrBlocks
  where
+  k = ledgerDbCfgSecParam $ ldbCfg env
+
   takeSnapshot ::
     CodecConfig blk ->
     Tracer m (TraceSnapshotEvent blk) ->