Expose the bottommost level compaction option to make it more flexible.

* *: reformat code

Just reformat code without other changes.

* travis: add cargo fmt

* travis: update rust version

* travis: use stable version

Sync of https://github.com/pingcap/rocksdb/pull/22

Fix the following gcc-8 warnings:
- conflicting C language linkage declaration [-Werror]
- writing to an object with no trivial copy-assignment [-Werror=class-memaccess]
- array subscript -1 is below array bounds [-Werror=array-bounds]

* change block cache capacity

* Add wrapper for CompactFiles

* modify external sst file's global seq no