Check/update API to correctly convey tx/block/layer finality

[lrettig]

The API was originally designed to convey several degrees of confirmation/finality, but a few things have changed since it was designed and implemented, including the introduction of the Hare certificate.

Per @tal-m, we consider a layer/block/tx final if:

• a block has a Hare certificate. the certificate is needed only for sync; if a node observed the Hare protocol completing successfully, that's equivalent.
• or, if the Hare fails, or we're in self-healing mode, it's enough that the vote margin for a block passes the global confidence threshold.

Note that, just like Bitcoin, we don't have actual finality. The "finality guidelines" above guarantee irreversibility except with 2^{-40} probability, assuming all the security assumptions hold.

to be on the safe side it's probably better to wait until the global threshold is reached even when we see a Hare output.

[lrettig]

I spent some time refamiliarizing myself with the tx/layer flow and how the API reports it. For reference a lot of the history is explained/linked here: spacemeshos/api#144.

Right now the node applies a layer after the Hare outputs a block. At this point the layer gets marked LAYER_STATUS_APPLIED. There's no differentiation at present between Hare and Tortoise "approving" a tx/block/layer. TransactionState and TransactionResult and TransactionReceipt don't contain any information on this, either: the TransactionResult.layer gets set when the tx gets processed/applied to state.

This needs some more thought/design.