Shrink replay buffer (#1616)

per
#1553 (comment)
and
3712da4

Author: mkeeter

upstairs/src/lib.rs

@@ -108,7 +108,7 @@ pub const IO_OUTSTANDING_MAX_JOBS: usize = 1000;
 ///
 /// Caching complete jobs allows us to replay them if a Downstairs goes offline
 /// them comes back.
-const IO_CACHED_MAX_BYTES: u64 = 1024 * 1024 * 1024; // 1 GiB
+const IO_CACHED_MAX_BYTES: u64 = 64 * 1024 * 1024; // 64 MiB
 
 /// Maximum of jobs to cache from complete (but un-flushed) IO
 ///