Summary:
TryMerge() overzealously creates one huge file read request in an attempt to merge smaller disjoint requests. For example, ~30 input requests of ~100 bytes output as 1 request of 100 MiB causing alarmingly large read throughputs to be repeatedly observed by the environment.
Signed-off-by: Jason Volk <jason@zemos.net>

Pull Request resolved: https://github.com/facebook/rocksdb/pull/6979

Reviewed By: siying

Differential Revision: D22668892

Pulled By: cheng-chang

fbshipit-source-id: 7506fe9621b7f1a747dadf6b8ddb1b1a141c1937