feat: support msync for windows

[chux0519]

Hi, I encountered a windows power loss caused problem while using badger, and see that msync on windows is not supported.

I add the implementation and test it in a windows10 VM, it just works.

see: https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-flushviewoffile

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This change is 

[CLAassistant]

All committers have signed the CLA.

[maddie]

@chux0519 Would you please sign the CLA to let this PR go forward? Thanks!

[chux0519]

I’ve checked this already,and I’m pretty sure the email is correct,should I recommit and push again

[NamanJain8]

Nice! Thanks for the PR. 🎉

[NamanJain8]

@chux0519 Can you try re-signing the CLA, please? Maybe from the PC this time.

[chux0519]

@chux0519 Can you try re-signing the CLA, please? Maybe from the PC this time.

same, the CLA seem's doesn't know me
Should I change the user.name of git to chux0519 (currently is Yongsheng Xu) as showen in the picture and recommit again?

[NamanJain8]

Yeah true, you have made a commit as Yongsheng Xu. That's why the CLA is complaining. I think if you remove that commit and push the changes using key-based (and not username password) access to Github, it would accept it.

[chux0519]

I pushed the commits via ssh not user-password based method Anyway I will recommit as chux0519 and do a force push later when I came home

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________________________________ 发件人: Naman Jain ***@***.***> 发送时间: Tuesday, September 28, 2021 7:28:11 PM 收件人: dgraph-io/ristretto ***@***.***> 抄送: Yongsheng Xu ***@***.***>; Mention ***@***.***> 主题: Re: [dgraph-io/ristretto] feat: support msync for windows (#283) Yeah true, you have made a commit as Yongsheng Xu. That's why the CLA is complaining. I think if you use push a commit using key-based (and not username password) access to Github, it would accept it. ― You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub<#283 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ADM5S2USQTGG4HN724TUC33UEGREXANCNFSM5DR2LLWQ>. Triage notifications on the go with GitHub Mobile for iOS<https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android<https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.