Merge #190: Make 'threshold' and 'possible' BIP9 statistics optional

6538851 Make 'threshold' and 'possible' BIP9 statistics optional (Roman Zeyde)

Pull request description:

  Following bitcoin/bitcoin#21934, latest Bitcoin Core `getblockchaininfo` RPC does not return the `threshold` and `possible` keys after a softwork is locked in:
  ```
  $ bitcoin-cli getblockchaininfo | jq .softforks.taproot
  {
    "type": "bip9",
    "bip9": {
      "status": "locked_in",
      "bit": 2,
      "start_time": 1619222400,
      "timeout": 1628640000,
      "since": 687456,
      "statistics": {
        "period": 2016,
        "elapsed": 1721,
        "count": 1718
      },
      "min_activation_height": 709632
    },
    "active": false
  }
  ```

ACKs for top commit:
  RCasatta:
    ACK 6538851

Tree-SHA512: bc8038c60c5814f2579b9c8860bccd84419e3a58121ee4ab356d9412a15e073efbe91029541b27ccb0a9430e1e6d579d33ffcf07c5b32a133345df35bbfba958

Author: RCasatta

json/src/lib.rs

@@ -582,10 +582,10 @@ pub enum Bip9SoftforkStatus {
 #[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
 pub struct Bip9SoftforkStatistics {
     pub period: u32,
-    pub threshold: u32,
+    pub threshold: Option<u32>,
     pub elapsed: u32,
     pub count: u32,
-    pub possible: bool,
+    pub possible: Option<bool>,
 }
 
 #[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]