Don't use the LMDB useWritemap option if we know we don't need it

[FUDCo]

The LMDB useWritemap option, which was required for LMDB to work properly when running on WSL, unconditionally creates a ~2GB database file (even though it doesn't actually take up that much space on disk until you actually put that much data in). This makes it hard to measure database space usage by looking at the file size, since now the file size is constant. However, if we know we are running on an OS where we know LMDB works without the useWritemap option, we can do without it and return to a world where we can get useful measurements of database space usage. MacOS is one such, so at a minimum we can exempt that, which is what this change does. It's possible that we could cook up some more sophisticated way to test for WSL specifically, but this much simpler test is doable immediately.

[warner]

looks good, but let's add a comment to remind ourselves that macOS and linux are (probably) equally capable, it's just that os.platform doesn't give us a way to distinguish between linux and WSL