offload initial iterator seeks to threadpool

Signed-off-by: Jason Volk <jason@zemos.net>

src/database/stream/items_rev.rs

@@ -1,4 +1,4 @@
-use std::{pin::Pin, sync::Arc};
+use std::{convert, pin::Pin, sync::Arc};
 
 use conduit::Result;
 use futures::{
@@ -16,9 +16,17 @@ pub(crate) struct ItemsRev<'a> {
 }
 
 impl<'a> ItemsRev<'a> {
-	pub(crate) fn new(db: &'a Arc<Engine>, cf: &'a Arc<ColumnFamily>, opts: ReadOptions, from: From<'_>) -> Self {
+	pub(crate) fn new(db: &'a Arc<Engine>, cf: &'a Arc<ColumnFamily>, opts: ReadOptions) -> Self {
 		Self {
-			state: State::new(db, cf, opts).init_rev(from),
+			state: State::new(db, cf, opts),
+		}
+	}
+}
+
+impl<'a> convert::From<State<'a>> for ItemsRev<'a> {
+	fn from(state: State<'a>) -> Self {
+		Self {
+			state,
 		}
 	}
 }
@@ -30,6 +38,13 @@ impl<'a> Cursor<'a, KeyVal<'a>> for ItemsRev<'a> {
 
 	#[inline]
 	fn seek(&mut self) { self.state.seek_rev(); }
+
+	#[inline]
+	fn init(self, from: From<'a>) -> Self {
+		Self {
+			state: self.state.init_rev(from),
+		}
+	}
 }
 
 impl<'a> Stream for ItemsRev<'a> {