Summary:
Use autovector in MergingIterator so that if there are 4 or less child iterators in it, iterator wrappers are inline, which is more likely to be cache friendly.
Based on one test run with a shadow traffic of one product, it reduces CPU of MergingIterator::Seek() by half.
Test Plan: make all check
Reviewers: haobo, yhchiang, igor, dhruba
Reviewed By: igor
CC: leveldb
Differential Revision: https://reviews.facebook.net/D18531
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