rpc: make uptime monotonic across NTP jumps

Compute `uptime` from `SteadyClock` so it is unaffected by system time changes after startup.

Derive GUI startup time by subtracting the monotonic uptime from the wall clock time.

Add a functional test covering a large `setmocktime` jump.

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>

src/qt/clientmodel.cpp

@@ -210,7 +210,7 @@ bool ClientModel::isReleaseVersion() const
 
 QString ClientModel::formatClientStartupTime() const
 {
-    return QDateTime::fromSecsSinceEpoch(GetStartupTime()).toString();
+    return QDateTime::currentDateTime().addSecs(-TicksSeconds(GetUptime())).toString();
 }
 
 QString ClientModel::dataDir() const