Summary:
Added log_readahead_size option to control prefetching for Log::Reader.
This is mostly useful for reading a remotely located log, as it can save the number of round-trips when reading it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5592

Differential Revision: D16362989

Pulled By: elipoz

fbshipit-source-id: c5d4d5245a44008cd59879640efff70c091ad3e8