Summary:
db_stress doesn't cover upper or lower bound in iterators. Try to cover it by randomly assigning a random one. Also in prefix scan tests, with 50% of the chance, set next prefix as the upper bound.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4162
Differential Revision: D8953507
Pulled By: siying
fbshipit-source-id: f0f04e9cb6c07cbebbb82b892ca23e0daeea708b