Revert "implemented !Send for PerfContext struct (#442)" (#444)

This reverts commit 9abbf010.

As commentted in https://github.com/tikv/rust-rocksdb/pull/442#issuecomment-595001970, `PerfContext` is naturally `!Send` because it contains a raw pointer.

Reverting #442 makes this crate compile on stable Rust.

![image](https://user-images.githubusercontent.com/17217495/75947377-1fa55880-5edb-11ea-8b20-b9e81ad7ac1a.png

)

Signed-off-by: Yilin Chen <sticnarf@gmail.com>