accept any new sidechain block from authorized enclave for shard

[brenzi]

closes #235

the previous logic where validateers had to announce their next block to be expected was too brittle and had no effective security benefit. All we need is:

• only authorized enclaves may finalize
• block numbers must increase monotonically
• avoid forks

avoiding forks is still the naive solution: just finalize blocks from the first validateer appearing in the ShardStatus, which usually is the primary validateer who created the shard

[clangenb]

Hmm, I see the point of avoiding forks by choosing which validateer is authorized to have its blocks finalized, and we can already say that having a round robing scheme for this authorization, we kind of ensure quick finality, if we don't have many validateers.

Might have been overengineered before.