Summary:
In case it is found that a key is already marked as locked in a
stripe's map of locked keys, it is not necessary to look it up
again using `std::unordered_map<std::string, ...>::at(size_t)`.
Instead, we can use the already found position using the iterator
produced by the previous `find` operation. Reusing the iterator
will avoid having to hash the key again and do additional "random"
memory lookups in the map of keys (though the data will very
likely sit available in caches here already due to the previous
find operation)
Closes https://github.com/facebook/rocksdb/pull/3505
Differential Revision: D7036446
Pulled By: sagar0
fbshipit-source-id: cced51547b2bd2d49394f6bc8c5896f09fa80f68
e9c31ab159