Adds an `obfuscate` parameter to `CLevelDBWrapper` and makes use of it
for all new chainstate stores built via `CCoinsViewDB`. Also adds an
`Xor` method to `CDataStream`.
Thanks to @sipa @laanwj @pstratem @dexX7 @KyrosKrane @gmaxwell.
This makes it easier for us to replace it if desired, since it's now only in
one spot. Also, it avoids the openssl include from allocators.h, which
essentially forced openssl to be included from every compilation unit.
