halve memory usage from state caches

beacon_chain/block_pools/candidate_chains.nim

@@ -356,11 +356,14 @@ func putStateCache(
   # with the concomitant memory allocator and GC load. Instead, use a
   # more memory-intensive (but more conceptually straightforward, and
   # faster) strategy to just store, for the most recent slots.
+  if state.data.slot mod 2 != 0:
+    return
+
   let stateCacheIndex = dag.getStateCacheIndex(blck.root, state.data.slot)
   if stateCacheIndex == -1:
     # Could use a deque or similar, but want simpler structure, and the data
     # items are small and few.
-    const MAX_CACHE_SIZE = 32
+    const MAX_CACHE_SIZE = 16
 
     let cacheLen = dag.cachedStates.len
     doAssert cacheLen <= MAX_CACHE_SIZE

beacon_chain/spec/beaconstate.nim

@@ -174,12 +174,12 @@ proc slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex,
     validator.effective_balance div MIN_SLASHING_PENALTY_QUOTIENT)
 
   # The rest doesn't make sense without there being any proposer index, so skip
-  # Apply proposer and whistleblower rewards
   let proposer_index = get_beacon_proposer_index(state, cache)
   if proposer_index.isNone:
     debug "No beacon proposer index and probably no active validators"
     return
 
+  # Apply proposer and whistleblower rewards
   let
     # Spec has whistleblower_index as optional param, but it's never used.
     whistleblower_index = proposer_index.get