[Compaction] Include being_compacted into SstFileMetaData

Summary:
Include being_compacted into SstFileMetaData.

This diff will be pushed into compaction branch.

Test Plan:
export ROCKSDB_TESTS=MetaData
make db_test -j32
./db_test

Reviewers: sdong, ljin, igor, dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21957