Hotcold: Changing compaction based on hotness of record.

Hotcold: Changing compaction based on hotness of record.

Summary:
Changed `DoCompactionWork()` to output multiple files and imported https://reviews.facebook.net/D8025 (Ideally all issues with version_set.h, version_set.cc and version_set_test.cc should be brought up there so I can keep that patch up to date and we can decide whether to mainline it separately) so we can handle overlapping files in level 1+.

Added code to determine whether a record is hot or cold given the metrics DB and an iterator on the database (which allows us to get all the necessary data in order to lookup the hotness)

Test Plan:
make check

db_bench --benchmarks=readrandomwriterandom --hotcold=1

Reviewers: dhruba, vamsi, chip, heyongqiang

Reviewed By: vamsi

CC: leveldb
Differential Revision: https://reviews.facebook.net/D8163