This CL switches the public headers to C++11 default and deleted constructors, and adds override to the relevant leveldb::EnvWrapper methods. This should be a good test for C++11 compiler support.

Once this CL settles, the rest of the codebase can be safely modernized to C++11.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189873212