feat: make block timestamp repr time block found

Closes #149.

The mining loop now updates block_header.timestamp with current time
before calculating each digest.  This means that the timestamp will
represent the moment a block is found, rather than the moment mining
started, as it was before.

Changes:
 * adds Block::set_header_timestamp()
 * call Block::set_header_timestamp(Timestamp::now()) in mining loop

The test `block_timestamp_represents_time_block_found()` now passes.

src/mine_loop.rs

@@ -179,6 +179,12 @@ fn mine_block_worker(
         // mutate nonce in the block's header.
         // Block::hash() will subsequently return a new digest.
         block.set_header_nonce(rng.gen());
+
+        // See issue #149 and test block_timestamp_represents_time_block_found()
+        // this ensures header timestamp represents the moment block is found.
+        // this is simplest impl.  Efficiencies can perhaps be gained by only
+        // performing every N iterations, or other strategies.
+        block.set_header_timestamp(Timestamp::now());
     }
 
     let nonce = block.kernel.header.nonce;
@@ -669,6 +675,9 @@ mod mine_loop_tests {
     ///
     /// This is a regression test for issue #149.
     /// https://github.com/Neptune-Crypto/neptune-core/issues/149
+    ///
+    /// note: this test fails in 318b7a20baf11a7a99f249660f1f70484c586012
+    ///       and should always pass in later commits.
     #[traced_test]
     #[tokio::test]
     async fn block_timestamp_represents_time_block_found() -> Result<()> {

src/models/blockchain/block/mod.rs

@@ -196,11 +196,21 @@ impl Block {
     /// sets header header nonce.
     ///
     /// note: this causes block digest to change.
+    #[inline]
     pub fn set_header_nonce(&mut self, nonce: [BFieldElement; 3]) {
         self.kernel.header.nonce = nonce;
         self.unset_digest();
     }
 
+    /// sets header timestamp.
+    ///
+    /// note: this causes block digest to change.
+    #[inline]
+    pub fn set_header_timestamp(&mut self, timestamp: Timestamp) {
+        self.kernel.header.timestamp = timestamp;
+        self.unset_digest();
+    }
+
     #[inline]
     pub fn header(&self) -> &BlockHeader {
         &self.kernel.header