Fasterlog lowmem (#178)

Adding support for low memory footprint (4 pages)
Added support for odd-sized payloads in presence of holes in log
Fixed concurrency issue that occurs with low num of pages
Improved max throughput by eliminating a 10ms sleep in BlockAllocate
Misc cleanup of logic to track flush and close addresses in log

cs/playground/FasterLogSample/Program.cs

@@ -51,7 +51,7 @@ static void AppendThread()
             while (true)
             {
                 log.Append(entry);
-                
+
                 // We also support a Span-based version of Append
 
                 // We also support TryAppend to allow throttling/back-off:
@@ -84,7 +84,7 @@ static void ScanThread()
                         Thread.Sleep(1000);
                     if (!result.SequenceEqual(entrySpan))
                     {
-                        throw new Exception("Invalid entry found");
+                        throw new Exception("Invalid entry found at offset " + FindDiff(result, entrySpan));
                     }
 
                     if (r.Next(100) < 10)
@@ -99,6 +99,18 @@ static void ScanThread()
             }
         }
 
+        static int FindDiff(Span<byte> b1, Span<byte> b2)
+        {
+            for (int i=0; i<b1.Length; i++)
+            {
+                if (b1[i] != b2[i])
+                {
+                    return i;
+                }
+            }
+            return 0;
+        }
+
         static void Main(string[] args)
         {
             var device = Devices.CreateLogDevice("D:\\logs\\hlog.log");