Snapshots are stored in the LMDB store not tar files

This has a few advantages:
 1. It makes the Bazel remote execution API much easier to implement.
 2. We store each unique file by content exactly one time, rather than
    once per snapshot it belongs to.

It also allows us to delete a lot of code which handles awkward
specifics of tar files.

src/rust/engine/fs/Cargo.toml

@@ -22,7 +22,6 @@ lmdb = "0.7.2"
 ordermap = "0.2.8"
 protobuf = "1.4.1"
 sha2 = "0.6.0"
-tar = "0.4.13"
 tempdir = "0.3.5"
 
 [dev-dependencies]

src/rust/engine/fs/fs_util/src/main.rs

@@ -8,7 +8,7 @@ extern crate protobuf;
 
 use boxfuture::{Boxable, BoxFuture};
 use clap::{App, Arg, SubCommand};
-use fs::{GetFileDigest, ResettablePool, Snapshot, Store, VFS};
+use fs::{ResettablePool, Snapshot, Store, StoreFileByDigest, VFS};
 use futures::future::{self, Future, join_all};
 use hashing::{Digest, Fingerprint};
 use protobuf::Message;
@@ -206,7 +206,7 @@ fn execute(top_match: clap::ArgMatches) -> Result<(), ExitError> {
               let digest = FileSaver {
                 store: store,
                 posix_fs: Arc::new(posix_fs),
-              }.digest(&f)
+              }.store_by_digest(&f)
                 .wait()
                 .unwrap();
               Ok(println!("{} {}", digest.0, digest.1))
@@ -246,14 +246,14 @@ fn execute(top_match: clap::ArgMatches) -> Result<(), ExitError> {
             .and_then(move |paths| {
               Snapshot::from_path_stats(
                 store.clone(),
-                Arc::new(FileSaver {
+                FileSaver {
                   store: store.clone(),
                   posix_fs: posix_fs,
-                }),
+                },
                 paths,
               )
             })
-            .map(|snapshot| snapshot.digest.unwrap())
+            .map(|snapshot| snapshot.digest)
             .wait()?;
           Ok(println!("{} {}", digest.0, digest.1))
         }
@@ -331,13 +331,14 @@ fn make_posix_fs<P: AsRef<Path>>(root: P, pool: Arc<ResettablePool>) -> fs::Posi
   fs::PosixFS::new(&root, pool, vec![]).unwrap()
 }
 
+#[derive(Clone)]
 struct FileSaver {
   store: Arc<Store>,
   posix_fs: Arc<fs::PosixFS>,
 }
 
-impl GetFileDigest<String> for FileSaver {
-  fn digest(&self, file: &fs::File) -> BoxFuture<Digest, String> {
+impl StoreFileByDigest<String> for FileSaver {
+  fn store_by_digest(&self, file: &fs::File) -> BoxFuture<Digest, String> {
     let file_copy = file.clone();
     let store = self.store.clone();
     self