L0 compression should follow options.compression_per_level if not empty
L0 compression should follow options.compression_per_level if not empty

Summary:
Currently, we don't use options.compression_per_level[0] as the compression style for L0 compression type, unless it is None. This behavior
 doesn't look like on purpose. This diff will make sure L0 compress using the style of options.compression_per_level[0].

Reviewed and accepted in: https://reviews.facebook.net/D65607
Closes https://github.com/facebook/rocksdb/pull/1435

Differential Revision: D4099368

Pulled By: siying

fbshipit-source-id: cfbbdcd

SunguckLee/RocksDB,wenduo/rocksdb,vmx/rocksdb,Andymic/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wenduo/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,bbiao/rocksdb,bbiao/rocksdb,wenduo/rocksdb,wenduo/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,vmx/rocksdb,wenduo/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB