Summary:
Three small optimizations:
(1) iter_->IsKeyPinned() shouldn't be called if read_options.pin_data is not true. This may trigger function call all the way down the iterator tree.
(2) reuse the iterator key object in DBIter::FindNextUserEntryInternal(). The constructor of the class has some overheads.
(3) Move the switching direction logic in MergingIterator::Next() to a separate function.

These three in total improves readseq performance by about 3% in my benchmark setting.
Closes https://github.com/facebook/rocksdb/pull/2880

Differential Revision: D5829252

Pulled By: siying

fbshipit-source-id: 991aea10c6d6c3b43769cb4db168db62954ad1e3