Make some MemoryPoolBlock operations volatile

- Making MemoryPoolBlock.End volatile and using Volatile.Write for
  MemoryPoolBlock.Next prevents a race that could cause threads consuming
  blocks to skip produced bytes.

src/Microsoft.AspNetCore.Server.Kestrel/Http/SocketInput.cs

@@ -105,7 +105,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
                     }
                     else
                     {
-                        _tail.Next = _pinned;
+                        Volatile.Write(ref _tail.Next, _pinned);
                         _tail = _pinned;
                     }
 

src/Microsoft.AspNetCore.Server.Kestrel/Infrastructure/MemoryPoolBlock.cs

@@ -57,14 +57,14 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Infrastructure
         /// the Start is guaranteed to be equal to Array.Offset. The value of Start may be assigned anywhere between Data.Offset and
         /// Data.Offset + Data.Count, and must be equal to or less than End.
         /// </summary>
-        public int Start { get; set; }
+        public int Start;
 
         /// <summary>
         /// The End represents the offset into Array where the range of "active" bytes ends. At the point when the block is leased
         /// the End is guaranteed to be equal to Array.Offset. The value of Start may be assigned anywhere between Data.Offset and
         /// Data.Offset + Data.Count, and must be equal to or less than End.
         /// </summary>
-        public int End { get; set; }
+        public volatile int End;
 
         /// <summary>
         /// Reference to the next block of data when the overall "active" bytes spans multiple blocks. At the point when the block is
@@ -72,7 +72,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Infrastructure
         /// working memory. The "active" memory is grown when bytes are copied in, End is increased, and Next is assigned. The "active" 
         /// memory is shrunk when bytes are consumed, Start is increased, and blocks are returned to the pool.
         /// </summary>
-        public MemoryPoolBlock Next { get; set; }
+        public MemoryPoolBlock Next;
 
         ~MemoryPoolBlock()
         {

src/Microsoft.AspNetCore.Server.Kestrel/Infrastructure/MemoryPoolIterator.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Diagnostics;
 using System.Numerics;
+using System.Threading;
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Infrastructure
 {
@@ -767,7 +768,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Infrastructure
                 {
                     var nextBlock = pool.Lease();
                     block.End = blockIndex;
-                    block.Next = nextBlock;
+                    Volatile.Write(ref block.Next, nextBlock);
                     block = nextBlock;
 
                     blockIndex = block.Data.Offset;
@@ -820,7 +821,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Infrastructure
                     {
                         var nextBlock = pool.Lease();
                         block.End = blockIndex;
-                        block.Next = nextBlock;
+                        Volatile.Write(ref block.Next, nextBlock);
                         block = nextBlock;
 
                         blockIndex = block.Data.Offset;