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/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())
+}