tests and fixes for Writer and Syncer

* separate these out into a new file, writer.rs, as dir.rs was getting
  unwieldy.
* extract traits for the parts of SampleFileDir and std::fs::File they needed;
  set up mock implementations.
* move clock.rs to a new base crate to be accessible from the db crate.
* add tests that exercise all the retry paths.
* bugfix: account for the new recording's bytes when calculating how much to
  delete.
* bugfix: when retrying an unlink failure in collect_garbage, we shouldn't
  warn about all the recordings no longer existing. Do this by retrying each
  step rather than the whole procedure again.
* avoid double-panic scenarios, which I hit while tweaking the mocks. These
  are quite annoying to debug as Rust doesn't print information about either
  panic. I ended up using lldb to get a backtrace. Better to be cautious about
  what we're doing when already panicking.
* give more context on raw::insert_recording errors, which I hit as well while
  tweaking the new tests.

src/cmds/config/cameras.rs

@@ -33,7 +33,7 @@ extern crate cursive;
 use self::cursive::Cursive;
 use self::cursive::traits::{Boxable, Identifiable, Finder};
 use self::cursive::views;
-use db::{self, dir};
+use db::{self, writer};
 use failure::Error;
 use std::collections::BTreeMap;
 use std::str::FromStr;
@@ -188,7 +188,7 @@ fn confirm_deletion(siv: &mut Cursive, db: &Arc<db::Database>, id: i32, to_delet
                         None => continue,
                     };
                     let l = zero_limits.entry(dir_id).or_insert_with(|| Vec::with_capacity(2));
-                    l.push(dir::NewLimit {
+                    l.push(writer::NewLimit {
                         stream_id,
                         limit: 0,
                     });
@@ -209,12 +209,12 @@ fn confirm_deletion(siv: &mut Cursive, db: &Arc<db::Database>, id: i32, to_delet
     }
 }
 
-fn lower_retention(db: &Arc<db::Database>, zero_limits: BTreeMap<i32, Vec<dir::NewLimit>>)
+fn lower_retention(db: &Arc<db::Database>, zero_limits: BTreeMap<i32, Vec<writer::NewLimit>>)
                    -> Result<(), Error> {
     let dirs_to_open: Vec<_> = zero_limits.keys().map(|id| *id).collect();
     db.lock().open_sample_file_dirs(&dirs_to_open[..])?;
     for (&dir_id, l) in &zero_limits {
-        dir::lower_retention(db.clone(), dir_id, &l)?;
+        writer::lower_retention(db.clone(), dir_id, &l)?;
     }
     Ok(())
 }

src/cmds/config/dirs.rs

@@ -33,7 +33,7 @@ extern crate cursive;
 use self::cursive::Cursive;
 use self::cursive::traits::{Boxable, Identifiable};
 use self::cursive::views;
-use db::{self, dir};
+use db::{self, writer};
 use failure::Error;
 use std::cell::RefCell;
 use std::collections::BTreeMap;
@@ -142,7 +142,7 @@ fn actually_delete(model: &RefCell<Model>, siv: &mut Cursive) {
     let model = &*model.borrow();
     let new_limits: Vec<_> =
         model.streams.iter()
-             .map(|(&id, s)| dir::NewLimit {stream_id: id, limit: s.retain.unwrap()})
+             .map(|(&id, s)| writer::NewLimit {stream_id: id, limit: s.retain.unwrap()})
              .collect();
     siv.pop_layer();  // deletion confirmation
     siv.pop_layer();  // retention dialog
@@ -150,7 +150,7 @@ fn actually_delete(model: &RefCell<Model>, siv: &mut Cursive) {
         let mut l = model.db.lock();
         l.open_sample_file_dirs(&[model.dir_id]).unwrap();  // TODO: don't unwrap.
     }
-    if let Err(e) = dir::lower_retention(model.db.clone(), model.dir_id, &new_limits[..]) {
+    if let Err(e) = writer::lower_retention(model.db.clone(), model.dir_id, &new_limits[..]) {
         siv.add_layer(views::Dialog::text(format!("Unable to delete excess video: {}", e))
                       .title("Error")
                       .dismiss_button("Abort"));