Update upstream to v5.17.2, and add raw bindings for new C API.

[arsing]

Ref #51

I've verified the RocksDbSharpTest and our own application's internal tests on Linux and Windows pass with this.

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TODO before merging:

• [ ] Test on Windows and Mac. Can you help with this? I don't have easy access to a Mac machine.

• [ ] Switch git repos in build script back to warrenfalk forks and switch tagref (v5.17.2) back to an explicit commit hash, once the forks been synced with upstream. Can you update your rocksdb and snappy forks please?

• [ ] Update download-native.cmd and download-native.sh to point to new zip downloads.

[warrenfalk]

Thanks!

I'll get a build of this published soon.

I was able to get this to build on a Mac except the 32 bit build on Mac is broken and I'm not sure it's fixable, so with this next release, I'm planning to just drop 32-bit native support. Rocksdb-sharp should still support a 32-bit native library if it is installed on the system.