Minimize accessing multiple objects in Version::Get()
Summary: One of our profilings shows that Version::Get() sometimes is slow when getting pointer of user comparators or other global objects. In this patch: (1) we keep pointers of immutable objects in Version to avoid accesses them though option objects or cfd objects (2) table_reader is directly cached in FileMetaData so that table cache don't have to go through handle first to fetch it (3) If level 0 has less than 3 files, skip the filtering logic based on SST tables' key range. Smallest and largest key are stored in separated memory locations, which has potential cache misses Test Plan: make all check Reviewers: haobo, ljin Reviewed By: haobo CC: igor, yhchiang, nkg-, leveldb Differential Revision: https://reviews.facebook.net/D17739
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