fix countdown over time period (#4449)

*sigh*

fixes:
```
Error: unhandled exception:
beacon_chain/beacon_chain_db_light_client.nim(214, 12)
`period.isSupportedBySQLite`  [AssertionDefect]
```

beacon_chain/spec/beacon_time.nim

@@ -66,6 +66,11 @@ template ethTimeUnit*(typ: type) {.dirty.} =
   proc `div`*(x: uint64, y: typ): uint64 {.borrow, noSideEffect.}
   proc `-`*(x: typ, y: typ): uint64 {.borrow, noSideEffect.}
 
+  iterator countdown*(a, b: typ, step: Positive = 1): typ =
+    # otherwise we use the signed version that breaks at the boundary
+    for i in countdown(distinctBase(a), distinctBase(b), step):
+      yield typ(i)
+
   proc `*`*(x: typ, y: uint64): uint64 {.borrow, noSideEffect.}
 
   proc `+=`*(x: var typ, y: typ) {.borrow, noSideEffect.}

tests/test_beacon_time.nim

@@ -47,3 +47,10 @@ suite "Beacon time":
         Epoch(5).sync_committee_period
       SyncCommitteePeriod(5).start_slot.sync_committee_period ==
         SyncCommitteePeriod(5)
+
+    block:
+      var counts = 0
+      for i in countdown(SyncCommitteePeriod(1), SyncCommitteePeriod(0)):
+        counts += 1
+      check:
+        counts == 2