keep unviables in quarantine (#3331)

they remain unviable even after a reorg

beacon_chain/consensus_object_pools/block_quarantine.nim

@@ -171,8 +171,11 @@ func cleanupOrphans(quarantine: var Quarantine, finalizedSlot: Slot) =
   for k in toDel:
     quarantine.addUnviable k[0]
 
-func clearQuarantine*(quarantine: var Quarantine) =
-  quarantine = Quarantine()
+func clearAfterReorg*(quarantine: var Quarantine) =
+  ## Clear missing and orphans to start with a fresh slate in case of a reorg
+  ## Unviables remain unviable and are not cleared.
+  quarantine.missing.reset()
+  quarantine.orphans.reset()
 
 # Typically, blocks will arrive in mostly topological order, with some
 # out-of-order block pairs. Therefore, it is unhelpful to use either a

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -1344,7 +1344,8 @@ proc updateHead*(
       dag.onReorgHappened(data)
 
     # A reasonable criterion for "reorganizations of the chain"
-    quarantine.clearQuarantine()
+    quarantine.clearAfterReorg()
+
     beacon_reorgs_total_total.inc()
     beacon_reorgs_total.inc()
   else:
@@ -1510,9 +1511,6 @@ proc preInit*(
     tailBlock: ForkedTrustedSignedBeaconBlock) =
   # write a genesis state, the way the ChainDAGRef expects it to be stored in
   # database
-  # TODO probably should just init a block pool with the freshly written
-  #      state - but there's more refactoring needed to make it nice - doing
-  #      a minimal patch for now..
 
   logScope:
     genesisStateRoot = getStateRoot(genesisState)