[Performance Branch] Fix a bug of PlainTable when building indexes

Summary:
PlainTable now has a bug of the ordering of indexes for the prefixes in the same bucket. I thought std::map guaranteed key order but it didn't, probably because I didn't use it properly. But seems to me that we don't need to make extra sorting as input prefixes are already sorted. Found by problem by running leaf4 against plain table. Replace the map with a vector. It should performs better too.

After the fix, leaf4 unit tests are passing.

Test Plan:
run plain_table_db_test
Also going to run db_test with plain table in the uncommitted branch.

Reviewers: haobo, kailiu

Reviewed By: haobo

CC: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D14649