Make further properties multi-client and thread-safe

Move MovingPrimary, MovingSecondary, EarliestNonSlewingTime to
SharedResources, make all new properties thread-safe (atomic)
operations.

Meade.net/ThreadSafeDateTime.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Threading;
+
+namespace ASCOM.Meade.net
+{
+    public class ThreadSafeDateTime
+    {
+        private long _value;
+
+        public ThreadSafeDateTime(in DateTime value) => _value = DateTimeToLong(value);
+
+        public void Set(in DateTime value) => Interlocked.Exchange(ref _value, DateTimeToLong(value));
+
+        private static long DateTimeToLong(in DateTime value) => value.ToUniversalTime().Ticks;
+
+        public static implicit operator ThreadSafeDateTime(in DateTime value) => new ThreadSafeDateTime(value);
+
+        public static implicit operator DateTime(ThreadSafeDateTime @this) => new DateTime(Interlocked.Read(ref @this._value), DateTimeKind.Utc);
+    }
+}