Pass previous block timestamp to Membership.RequestOrderedCommittee

[talkol]

We currently provide blockHeight only for getting the committee:

RequestOrderedCommittee(ctx context.Context, blockHeight primitives.BlockHeight, randomSeed uint64) []primitives.MemberId

Since the committee is now based on election results stored in state storage, we need to make a call to virtual machine to get the values

Calls to virtual machine require both block height and block timestamp

The block height is the last committed block (meaning the committee block height minus 1), but this means we need its timestamp too

If we have the header for the previous block to the committee that's ok too, the consensus context can extract the timestamp from there

@OdedWx @gadcl