make with_recording_playback less monomorphized

This is a minor code size reduction - instead of being monomorphized
into four variants (according to "cargo llvm-lines"), it's now
monomorphized into two. The stripped release binary on macOS is about
8kB smaller (0.15%). Not a huge improvement but better than nothing.

Benchmarks seem unchanged (though they have a lot of variance).

db/db.rs

@@ -1153,8 +1153,9 @@ impl LockedDatabase {
     /// Calls `f` with a single `recording_playback` row.
     /// Note the lock is held for the duration of `f`.
     /// This uses a LRU cache to reduce the number of retrievals from the database.
-    pub fn with_recording_playback<F, R>(&self, id: CompositeId, f: F) -> Result<R, Error>
-    where F: FnOnce(&RecordingPlayback) -> Result<R, Error> {
+    pub fn with_recording_playback<R>(&self, id: CompositeId,
+                                      f: &mut FnMut(&RecordingPlayback) -> Result<R, Error>)
+                                      -> Result<R, Error> {
         // Check for uncommitted path.
         let s = self.streams_by_id
                     .get(&id.stream())

db/recording.rs

@@ -394,7 +394,7 @@ impl Segment {
         // Slow path. Need to iterate through the index.
         trace!("recording::Segment::new slow path, desired_range_90k={:?}, recording={:#?}",
                self_.desired_range_90k, recording);
-        db.with_recording_playback(self_.id, |playback| {
+        db.with_recording_playback(self_.id, &mut |playback| {
             let mut begin = Box::new(SampleIndexIterator::new());
             let data = &(&playback).video_index;
             let mut it = SampleIndexIterator::new();
@@ -437,8 +437,9 @@ impl Segment {
             self_.video_sample_entry_id_and_trailing_zero =
                 recording.video_sample_entry_id |
                 (((it.duration_90k == 0) as i32) << 31);
-            Ok(self_)
-        })
+            Ok(())
+        })?;
+        Ok(self_)
     }
 
     pub fn video_sample_entry_id(&self) -> i32 {
@@ -632,7 +633,7 @@ mod tests {
     fn get_frames<F, T>(db: &db::Database, segment: &Segment, f: F) -> Vec<T>
     where F: Fn(&SampleIndexIterator) -> T {
         let mut v = Vec::new();
-        db.lock().with_recording_playback(segment.id, |playback| {
+        db.lock().with_recording_playback(segment.id, &mut |playback| {
             segment.foreach(playback, |it| { v.push(f(it)); Ok(()) })
         }).unwrap();
         v