`StateSyncDone` messages may fail to be delivered

[StephenButtolph]

Describe the bug

Prior to the ToEngine -> WaitForEvent VM refactor, the proposervm did not guarantee to deliver StateSyncDone messages in a timely manner. Specifically, after SetPreference was called on the proposervm, all messages were rate limited by the proposervm.

After the ToEngine -> WaitForEvent VM refactor, the VM returning StateSyncDone does not guarantee that it will be handled by the consensus engine. However, there is no mechanism for the VM to know when the message was successfully handled.

Expected behavior

The VM should be able to know when the consensus code has received the StateSyncDone message so that it can internally switch from returning StateSyncDone messages to PendingTxs messages.