bugfix: prefetch bug

[yasyzb]

Description

1.Fix panic in trie_prefetcher close
2.Fix trie_prefetcher abortLoop goroutine leak

Rationale

1.In ValidateState, there are three goroutines running concurrently, if the previous one return error, the upper caller function InsertChain would return and call it's defer function to close prefetcher, while the IntermediateRoot goroutine still run and when it done call its defer function to close prefetcher, this time the prefetcher is nil, so it will trigger a panic. So we should wait all the goroutines done in ValidateState to avoid this panic.
2.In InsertChain, if we enable diffsync, and insert two more blocks here, then all the blocks except the last one wound't do call StopPrefetcher, so these blocks' abortLoop goroutine will leak. So we should do the StopPrefetcher in IntermediateRoot to avoid this leak.