[BUG]: *DB.Sync should sync memtable in addition to value log

[mcfog]

What version of Badger are you using?

github.com/dgraph-io/badger/v3 v3.2103.5

What version of Go are you using?

No response

Have you tried reproducing the issue with the latest release?

Yes

What is the hardware spec (RAM, CPU, OS)?

What steps will reproduce the bug?

call *DB.Sync()

Expected behavior and actual result.

Both memtable('s wal file) and value log should be persisted by Sync call.

Additional information

No response

[mangalaman93]

This seems reasonable to me. If you could raise a PR with a test that proves the issue, that'd be awesome. Otherwise, we will try to get to it as soon as we can. Thanks