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

185a70d cherry-pick lower bound for iterator
0335b52 cherry-pick addfe1ef4b611b8ac47a24f8d7aad92a20d5cb0b Fix tombstone scans in SeekForPrev outside prefix
cbb6be8 cherry-pick lower-bound for ReadOptions

git-subtree-dir: librocksdb_sys/rocksdb
git-subtree-split: 185a70d50e047f48b8a0a6023efbd7a1d687b31f