Review CityHash usage and ensure buffers are aligned

The CityHash64 implementation in WiredTiger assumes that the data passed in is 64-byte aligned. If it isn't the computed hash can be invalid, since it will include some bytes of random content past the end of the last block.

We should ensure that all places in the code using our CityHash implementation are doing so safely, and that future usages are forced (or strongly encouraged) to do so too.