Merge bitcoin/bitcoin#34328: rpc: make uptime monotonic across NTP jumps

14f99cfe53 rpc: make `uptime` monotonic across NTP jumps (Lőrinc)
a9440b1595 util: add `TicksSeconds` (Lőrinc)

Pull request description:

  ### Problem
  `bitcoin-cli uptime` was derived from wall-clock time, so it could jump by large amounts when the system clock is corrected after `bitcoind` starts (e.g. on RTC-less systems syncing NTP).
  This breaks the expectation that uptime reflects process runtime.

  ### Fix
  Compute uptime from a [monotonic clock](https://en.cppreference.com/w/cpp/chrono/steady_clock.html) so it is immune to wall-clock jumps, and use that monotonic uptime for the RPC.
  GUI startup time is derived from wall clock time minus monotonic uptime so it remains sensible after clock corrections.

  ### Reproducer
  Revert the fix commit and run the `rpc_uptime` functional test (it should fail with `AssertionError: uptime should not jump with wall clock`):

  Or alternatively:

  ```bash
  cmake -B build && cmake --build build --target bitcoind bitcoin-cli -j$(nproc)
  DATA_DIR=$(mktemp -d)
  ./build/bin/bitcoind -regtest -datadir="$DATA_DIR" -connect=0 -daemon
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" -rpcwait uptime
  sleep 1
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" setmocktime $(( $(date +%s) + 20000000 ))
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" uptime
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" stop
  ```

  <details>
  <summary>Before (uptime jumps with wall clock)</summary>

  ```bash
  Bitcoin Core starting
  0
  20000001
  Bitcoin Core stopping
  ```

  </details>

  <details>
  <summary>After (uptime stays monotonic)</summary>

  ```bash
  Bitcoin Core starting
  0
  1
  Bitcoin Core stopping
  ```
  </details>

  ----------

  Issue: https://github.com/bitcoin/bitcoin/issues/34326

ACKs for top commit:
  maflcko:
    review ACK 14f99cfe53 🎦
  willcl-ark:
    tACK 14f99cfe53
  w0xlt:
    ACK 14f99cfe53
  sedited:
    ACK 14f99cfe53

Tree-SHA512: 3909973f58666ffa0b784a6df087031b9e34d2022d354900a4dbb6cbe1d36285cd92770ee71350ebf64d6e8ab212d8ff0cd851f7dca1ec46ee2f19b417f53984

test/functional/rpc_uptime.py

@@ -26,9 +26,12 @@ class UptimeTest(BitcoinTestFramework):
         assert_raises_rpc_error(-8, "Mocktime must be in the range [0, 9223372036], not -1.", self.nodes[0].setmocktime, -1)
 
     def _test_uptime(self):
-        wait_time = 10
-        self.nodes[0].setmocktime(int(time.time() + wait_time))
-        assert self.nodes[0].uptime() >= wait_time
+        wait_time = 20_000
+        uptime_before = self.nodes[0].uptime()
+        self.nodes[0].setmocktime(int(time.time()) + wait_time)
+        uptime_after = self.nodes[0].uptime()
+        self.nodes[0].setmocktime(0)
+        assert uptime_after - uptime_before < wait_time, "uptime should not jump with wall clock"
 
 
 if __name__ == '__main__':