Managedlocalstorage fixes (#209)

* Improved ManagedLocalStorageDevice to use ReadAsync and WriteAsync.
* Added fcntl calls for non-windows machines.
* On non-windows, serialize writes so that a second write is issued on handle only after previous write completes and enters completion callback. Without this restriction, we see incorrect data (zeros) flushed to the log. Needs further investigation in future.

cs/playground/FasterLogSample/Program.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Runtime.InteropServices;
 using System.Threading;
 using System.Threading.Tasks;
 using FASTER.core;
@@ -30,7 +31,13 @@ static void Main()
                 staticEntry[i] = (byte)i;
             }
 
-            var device = Devices.CreateLogDevice("D:\\logs\\hlog.log");
+            IDevice device;
+
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                device = Devices.CreateLogDevice("D:\\logs\\hlog.log");
+            else
+                device = Devices.CreateLogDevice("/mnt/tmp/hlog/hlog.log");
+
             log = new FasterLog(new FasterLogSettings { LogDevice = device });
 
             using (iter = log.Scan(log.BeginAddress, long.MaxValue))
@@ -150,15 +157,15 @@ static void ScanThread()
 
             while (true)
             {
-                while (!iter.GetNext(out result, out int length, out _))
+                while (!iter.GetNext(out result, out _, out _))
                 {
                     // For finite end address, check if iteration ended
                     // if (iter.CurrentAddress >= endAddress) return; 
                     iter.WaitAsync().GetAwaiter().GetResult();
                 }
 
                 // Memory pool variant:
-                // iter.GetNext(pool, out IMemoryOwner<byte> resultMem, out int length))
+                // iter.GetNext(pool, out IMemoryOwner<byte> resultMem, out int length, out long currentAddress)
 
                 if (Different(result, staticEntry))
                     throw new Exception("Invalid entry found");