Summary:
1. If expiration_time is non-positive, no need to call NowMicros, save a syscall.
2. expire_time should only be set when expired is false.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6586

Test Plan: make check

Reviewed By: lth

Differential Revision: D20673730

Pulled By: cheng-chang

fbshipit-source-id: a69e8d7b16dc6d0d00487bb1c19f0710d79482e2