Further decouple justification and finalisation (#958)

Addresses @djrtwo's comment [here](#925 (comment)).

specs/core/0_beacon-chain.md

@@ -1683,7 +1683,8 @@ def update_justification_and_finalization(state: BeaconState) -> None:
     if get_current_epoch(state) <= GENESIS_EPOCH + 1:
         return
 
-    antepenultimate_justified_epoch = state.previous_justified_epoch
+    old_previous_justified_epoch = state.previous_justified_epoch
+    old_current_justified_epoch = state.current_justified_epoch
 
     # Process justifications
     state.previous_justified_epoch = state.current_justified_epoch
@@ -1704,20 +1705,20 @@ def update_justification_and_finalization(state: BeaconState) -> None:
     bitfield = state.justification_bitfield
     current_epoch = get_current_epoch(state)
     # The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th as source
-    if (bitfield >> 1) % 8 == 0b111 and antepenultimate_justified_epoch == current_epoch - 3:
-        state.finalized_epoch = antepenultimate_justified_epoch
+    if (bitfield >> 1) % 8 == 0b111 and old_previous_justified_epoch == current_epoch - 3:
+        state.finalized_epoch = old_previous_justified_epoch
         state.finalized_root = get_block_root(state, get_epoch_start_slot(state.finalized_epoch))
     # The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as source
-    if (bitfield >> 1) % 4 == 0b11 and antepenultimate_justified_epoch == current_epoch - 2:
-        state.finalized_epoch = antepenultimate_justified_epoch
+    if (bitfield >> 1) % 4 == 0b11 and old_previous_justified_epoch == current_epoch - 2:
+        state.finalized_epoch = old_previous_justified_epoch
         state.finalized_root = get_block_root(state, get_epoch_start_slot(state.finalized_epoch))
     # The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as source
-    if (bitfield >> 0) % 8 == 0b111 and state.previous_justified_epoch == current_epoch - 2:
-        state.finalized_epoch = state.previous_justified_epoch
+    if (bitfield >> 0) % 8 == 0b111 and old_current_justified_epoch == current_epoch - 2:
+        state.finalized_epoch = old_current_justified_epoch
         state.finalized_root = get_block_root(state, get_epoch_start_slot(state.finalized_epoch))
     # The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source
-    if (bitfield >> 0) % 4 == 0b11 and state.previous_justified_epoch == current_epoch - 1:
-        state.finalized_epoch = state.previous_justified_epoch
+    if (bitfield >> 0) % 4 == 0b11 and old_current_justified_epoch == current_epoch - 1:
+        state.finalized_epoch = old_current_justified_epoch
         state.finalized_root = get_block_root(state, get_epoch_start_slot(state.finalized_epoch))
 ```