Added flushing OS cache to medium

[mkosieradzki]

This solves #574 . However only the for the portable version, because Flush(true) API has been added in .NET Framework 4 (and works properly since .NET Framework 4.5). So it will not work in .NET Framework 3.5.

[lidanger]

How about Flush(true) in .NET Framework 4? @mkosieradzki

It seems the bug was fixed until version 4.5 according to the following page:
https://connect.microsoft.com/VisualStudio/feedback/details/634385/filestream-flush-flushtodisk-true-call-does-not-flush-the-buffers-to-disk

[lidanger]

Can you compile the code to other version of the .NET Framework and add the library to the Nuget? @mbdavid

[mbdavid]

@lidanger, yes, both in .net35 and netstandard. But its implemented only for netstandard (so, will not work in .net 4)

#if !NET35
            _journal.Flush(true);
#endif