Add oldest_block in /network/status

[matiwinnetou]

As we start to use pruned modes or as it will be called soon: REMOVE_SPENT_UTXOS) more and more, we must report oldest_block in /network/status.

[linconvidal]

According to Mesh API specs:

If a Rosetta implementation prunes historical state, it should populate the optional oldest_block_identifier field with the oldest block available to query. If this is not populated, it is assumed that the genesis_block_identifier is the oldest queryable block.

Here is what the response looks like:

{
  "current_block_identifier": {
    "index": ...,
    "hash": "..."
  },
  "current_block_timestamp": ...,
  "genesis_block_identifier": {
    "index": 0,
    "hash": "..."
  },
  "oldest_block_identifier": {
    "index": ...,
    "hash": "..."
  },
  "sync_status": { ... },
  "peers": []
}

[matiwinnetou]

@linconvidal as discussed we will implement this as last full blocks based on "PRUNING_ENABLED" (REMOVE_SPENT_UTXOS) flag set.

A proper pruning according to definition from Mesh (as pruned from the genesis block we will implement in the future).