Summary:
Valgrind had false positive complaints about the initialization pattern for `GetCurrentTime()`'s argument in #2480. We can instead have the client initialize the time variable before calling `GetCurrentTime()`, and have `GetCurrentTime()` promise to only overwrite it in success case.
Closes https://github.com/facebook/rocksdb/pull/2526

Differential Revision: D5358689

Pulled By: ajkr

fbshipit-source-id: 857b189f24c19196f6bb299216f3e23e7bc4be42