Yi Wu 24a24f013d Enable configurable readahead for iterators
Summary:
Add an option `iterator_readahead_size` to `ReadOptions` to enable
configurable readahead for iterators similar to the corresponding
option for compaction.

Test Plan:
```
make commit_prereq
```

Reviewers: kumar.rangarajan, ott, igor, sdong

Reviewed By: sdong

Subscribers: yiwu, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D55419
2016-05-04 15:25:58 -07:00
..
2016-04-27 18:30:04 -07:00
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