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

@@ -18,7 +18,6 @@ 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

@@ -227,7 +227,7 @@ fn execute(top_match: clap::ArgMatches) -> Result<(), ExitError> {
                 paths,
               )
             })
-            .map(|snapshot| snapshot.digest.unwrap())
+            .map(|snapshot| snapshot.digest)
             .wait()?;
           Ok(println!("{} {}", digest.0, digest.1))
         }

src/rust/engine/fs/src/hash.rs

@@ -1,12 +1,8 @@
 // Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 // Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-use digest::{Digest, FixedOutput};
-use sha2::Sha256;
-
 use std::error::Error;
 use std::fmt;
-use std::io::{self, Write};
 
 use hex;
 
@@ -66,43 +62,6 @@ impl AsRef<[u8]> for Fingerprint {
   }
 }
 
-///
-/// A Write instance that fingerprints all data that passes through it.
-///
-pub struct WriterHasher<W: Write> {
-  hasher: Sha256,
-  inner: W,
-}
-
-impl<W: Write> WriterHasher<W> {
-  pub fn new(inner: W) -> WriterHasher<W> {
-    WriterHasher {
-      hasher: Sha256::default(),
-      inner: inner,
-    }
-  }
-
-  ///
-  /// Returns the result of fingerprinting this stream, and Drops the stream.
-  ///
-  pub fn finish(self) -> Fingerprint {
-    Fingerprint::from_bytes_unsafe(&self.hasher.fixed_result())
-  }
-}
-
-impl<W: Write> Write for WriterHasher<W> {
-  fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-    let written = self.inner.write(buf)?;
-    // Hash the bytes that were successfully written.
-    self.hasher.input(&buf[0..written]);
-    Ok(written)
-  }
-
-  fn flush(&mut self) -> io::Result<()> {
-    self.inner.flush()
-  }
-}
-
 #[cfg(test)]
 mod fingerprint_tests {
   use super::Fingerprint;