Add jemalloc feature (#319)

Adding feature "jemalloc". When enabled, rocksdb "WITH_JEMALLOC" will be enabled. This does NOT affect whether jemalloc is use or not. Mostly it only affect how rocksdb calculate block cache size: if "WITH_JEMALLOC" is on, it will use `malloc_usable_size()` (size allocated by jemalloc) to calculate size of each entry of block cache, instead of the requested size of the allocation.

Tested by running `cargo build --features jemalloc -vv` and check rocksdb compile flags from the output.

Signed-off-by: Yi Wu <yiwu@pingcap.com>