Update SstFileWriter to use bottommost_compression if avaliable
Update SstFileWriter to use bottommost_compression if avaliable

Summary: SstFileWriter ignore Options::bottommost_compression, update it to use bottommost_compression if available

Test Plan:
make check -j64
verified used compression using ./sst_dump

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, yoshinorim

Differential Revision: https://reviews.facebook.net/D59841

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