Summary:
In the initial stages of the windows port we had
problems building rocksdb on windows, so we disabled it.

These days we're able to build it and detect it--we even
require it in the cmake code, but hadn't gotten around
to telling the rest of the code that we can use it.

This commit re-enables it in the build but leaves sqlite
as the default engine until we're able to perform some
benchmarking.

Rocksdb itself has some build issues on Windows; it doesn't
use cmake to locate dependencies, so even though we built
snappy it doesn't know how to find it without modifying the
source:
https://github.com/facebook/rocksdb/blob/master/thirdparty.inc#L4

For that reason, we disable the use of Snappy in the Windows build.
However, in the version of rocksdb that we were using, it would
default to trying to use Snappy even though it wasn't compiled in
and throw an exception.

I've upgraded to a newer version of rocksdb that will simply not
use compression if no compression was enabled at build time.

Given that we mostly store relatively small objects, I'm assuming
that the lack of compression is fine for now.

Reviewed By: xavierd

Differential Revision: D21319896

fbshipit-source-id: 2a2d06d4bd5382706e9220f9b4a2de99dc18311d