avoid rewindStateData/state_transition in fast-path (#919)

* avoid rewindStateData/state_transition totally in fast-pathed updateStateData calls

* document motivation behind BlockPool.getStateDataCached(...) and clarify naming

beacon_chain/block_pool.nim

@@ -715,6 +715,28 @@ proc rewindState(pool: BlockPool, state: var StateData, bs: BlockSlot):
 
   ancestors
 
+proc getStateDataCached(pool: BlockPool, state: var StateData, bs: BlockSlot): bool =
+  # This pointedly does not run rewindState or state_transition, but otherwise
+  # mostly matches updateStateData(...), because it's too expensive to run the
+  # rewindState(...)/skipAndUpdateState(...)/state_transition(...) procs, when
+  # each hash_tree_root(...) consumes a nontrivial fraction of a second.
+  for db in [pool.db, pool.cachedStates[0], pool.cachedStates[1]]:
+    if (let tmp = db.getStateRoot(bs.blck.root, bs.slot); tmp.isSome()):
+      if not db.containsState(tmp.get):
+        continue
+
+      let
+        root = tmp.get()
+        ancestorState = db.getState(root)
+
+      doAssert ancestorState.isSome()
+      state.data.data = ancestorState.get()
+      state.data.root = root
+      state.blck = pool.get(bs.blck).refs
+      return true
+
+  false
+
 proc updateStateData*(pool: BlockPool, state: var StateData, bs: BlockSlot) =
   ## Rewind or advance state such that it matches the given block and slot -
   ## this may include replaying from an earlier snapshot if blck is on a
@@ -732,6 +754,9 @@ proc updateStateData*(pool: BlockPool, state: var StateData, bs: BlockSlot) =
 
     return # State already at the right spot
 
+  if pool.getStateDataCached(state, bs):
+    return
+
   let ancestors = rewindState(pool, state, bs)
 
   # If we come this far, we found the state root. The last block on the stack