Consolidate View and SeqNo typedefs

[eddyashton]

We currently have a lot of different typedefs for View and SeqNo (under kv::, kv::Consensus, etc), as well as some older terminology referring to Version, Index, and Term. As part of #697, the former should be consolidated into a single alias (ccf::View, ccf::SeqNo, from ccf/tx_id.h), and the latter should only be used inside the KV/Raft and not exposed beyond them.

[achamayou]

We should kill off all remaining usage of Term now that TxView is gone.

[eddyashton]

I'm making progress on this, with a few caveats:

• We were benefitting from a lot of direct conversions from kv::Version (signed so it can handle deletions) and SeqNo (unsigned). I think a lot of the uses of Version should actually be unsigned - wherever we use it to refer to an entire Tx for instance, it must be non-deleted. So I'm trending towards making these all unsigned, but maintaining a signed internal representation for deletions in the champ. This would be easier if we did Permanently delete items in the KV champ maps #1992 first, but I think we need to do it this way round to smooth off the API ASAP.
• This already includes a lot of hairy guesswork of "is this KV or everything-else", so I'm avoiding removing Term and Version terminology for now. Once they're all unsigned and the distinction is drawn, it should be easier to rename Term to View in isolation.