Consolidate RockDB code

Moves explicitly to zstd. RockDB recommends zstd, or at least lz4 over snappy,
and this minimizes which dependencies we pull in.

common/db/src/lib.rs

@@ -3,6 +3,8 @@ pub use mem::*;
 
 #[cfg(feature = "rocksdb")]
 mod rocks;
+#[cfg(feature = "rocksdb")]
+pub use rocks::{RocksDB, new_rocksdb};
 
 /// An object implementing get.
 pub trait Get {

common/db/src/rocks.rs

@@ -1,6 +1,6 @@
 use std::sync::Arc;
 
-use rocksdb::{ThreadMode, Transaction, TransactionDB};
+use rocksdb::{DBCompressionType, ThreadMode, SingleThreaded, Options, Transaction, TransactionDB};
 
 use crate::*;
 
@@ -32,3 +32,10 @@ impl<T: ThreadMode + 'static> Db for Arc<TransactionDB<T>> {
     self.transaction()
   }
 }
+
+pub type RocksDB = Arc<TransactionDB<SingleThreaded>>;
+pub fn new_rocksdb(path: &str) -> RocksDB {
+  let mut options = Options::default();
+  options.set_compression_type(DBCompressionType::Zstd);
+  Arc::new(TransactionDB::open(&options, &Default::default(), path).unwrap())
+}