Summary:
There were a few places where MSVC's implicit truncation warnings were getting triggered, which was causing the MSVC build to fail due to warnings being treated as errors. This resolves the issues by making the truncations in some places explicit, and by making it so there are no truncations of literals.
Fixes#3239
Supersedes #3259
Closes https://github.com/facebook/rocksdb/pull/3273
Reviewed By: yiwu-arbug
Differential Revision: D6569204
Pulled By: Orvid
fbshipit-source-id: c188cf1cf98d9acb6d94b71875041cc81f8ff088
b4d88d7128