Summary:
Right now RocksDB gets manifest file size before recovering from it. The information is available in LogReader. Use it instead to prevent one file system call.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6369

Test Plan: Run all existing tests

Differential Revision: D19714872

fbshipit-source-id: 0144be324d403c99e3da875ea2feccc8f64e883d