Venus: Message pool re-org handler should restore messages at height of new chain

[anorth]

Describe the bug:
On a new head we add messages into the message pool at the height they were mined at in the old chain. If the height of the newChain is greater than InboxMaxAgeTipsets when the message was added to the old chain, then the messages will be dropped immediately.

Expected behavior:
Messages should be restored to the pool at the height of the new chain, so that the relative heights of the old and new chain do not affect behaviour.

The outbox already behaves this way.

Reported by @travisperson

[anorth]

@acruikshank or @ingar could you please look into this? Should be a quick fix.