Make further properties multi-client and thread-safe

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

Meade.net/ThreadSafeNullableDouble.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Threading;
+
+namespace ASCOM.Meade.net
+{
+    public class ThreadSafeNullableDouble
+    {
+        private long _value;
+
+        public ThreadSafeNullableDouble(in double? value) => _value = NullableDoubleToLong(value);
+
+        public void Set(in double? value) => Interlocked.Exchange(ref _value, NullableDoubleToLong(value));
+
+        private static long NullableDoubleToLong(in double? value) => BitConverter.DoubleToInt64Bits(value ?? double.NaN);
+
+        public static implicit operator ThreadSafeNullableDouble(in double? value) => new ThreadSafeNullableDouble(value);
+
+        public static implicit operator double?(ThreadSafeNullableDouble @this)
+        {
+            var doubleValue = BitConverter.Int64BitsToDouble(Interlocked.Read(ref @this._value));
+            return double.IsNaN(doubleValue) ? null as double? : doubleValue;
+        }
+    }
+}