Problem: iterator on deeply nested cache contexts is extremely slow

Closes: evmos#616

Solution:
- flatten cache contexts before doing `GetTxLogsTransient`

x/evm/keeper/context_stack.go

@@ -60,6 +60,27 @@ func (cs *ContextStack) Commit() {
 	cs.cachedContexts = []cachedContext{}
 }
 
+// CommitToRevision flatter multiple layers of cache contexts into one layer,
+// to improve efficiency of db operations.
+func (cs *ContextStack) CommitToRevision(target int) {
+	if target < 0 || target >= len(cs.cachedContexts) {
+		panic(fmt.Errorf("snapshot index %d out of bound [%d..%d)", target, 0, len(cs.cachedContexts)))
+	}
+
+	targetCtx := cs.cachedContexts[target].ctx
+	// commit in order from top to bottom
+	for i := len(cs.cachedContexts) - 1; i > target; i-- {
+		// keep all the cosmos events
+		targetCtx.EventManager().EmitEvents(cs.cachedContexts[i].ctx.EventManager().Events())
+		if cs.cachedContexts[i].commit == nil {
+			panic(fmt.Sprintf("commit function at index %d should not be nil", i))
+		} else {
+			cs.cachedContexts[i].commit()
+		}
+	}
+	cs.cachedContexts = cs.cachedContexts[0 : target+1]
+}
+
 // Snapshot pushes a new cached context to the stack,
 // and returns the index of it.
 func (cs *ContextStack) Snapshot() int {

x/evm/keeper/state_transition.go

@@ -191,6 +191,9 @@ func (k *Keeper) ApplyTransaction(tx *ethtypes.Transaction) (*types.MsgEthereumT
 		return nil, stacktrace.Propagate(err, "failed to apply ethereum core message")
 	}
 
+	// flatten the cache contexts to improve efficiency of following db operations
+	k.ctxStack.CommitToRevision(revision)
+
 	k.IncreaseTxIndexTransient()
 
 	res.Hash = txHash.Hex()