Make set_snapshot public (simple pull request)

[a14e]

Good day!
I'm writing my database on top of rocks db. And I have a problem:
I want to cache snapshot (for consistency) iterator between different batch requests for stream processing, but current implementation requires lifetime and lifetime is incompatible with caching.
I'd like to do simple workaround via ReadOptions, but method set_snapshot is private. So I want to make it public =)

my current solution is:

let db: &'static DB = todo!();
let  column_family: &'static ColumnFamily = todo!();
let snapshot = db.snapshot();
let mut options = rocksdb::ReadOptions::default();
options.set_snapshot(&snapshot);
let iter = db.raw_iterator_cf_opt(column_family, options);