add checkpoint support for single db in regression test
add checkpoint support for single db in regression test

Summary:
For level_compaction_style regression test.
Closes https://github.com/facebook/rocksdb/pull/2397

Differential Revision: D5168545

Pulled By: lightmark

fbshipit-source-id: 195e4d84917e7c261d9f4fbe9aee5d104c9cb9a2

SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb