Avoid adding new proposer policy if there is no change

[arhag]

The host function should check to see if the policy to be proposed is different than the last proposal policy in the queue (not the currently active one). If it is the same, it should avoid actually adding the policy to the queue (it acts like a no-op).

The host function would uint32_t max as the version number if the proposed policy is not invalid. If it is invalid (e.g. empty producers in the policy) it would return -1; this is the only time is returns -1. This means that if the caller of the host function proposes a policy that is identical to the one it last proposed, the host function will still return successfully (not with a -1 return value).

The diff in the block extension is relative to the previous policy proposed and represented in a block extension (as a diff) in the history of the chain.

This is a change the protocol, so if we want to do it we need to get it done before rc1. But it also is not necessary to make this change to the protocol prior to release.