disk cache: store a data integrity header for non-CAS blobs

The header is made up of three fields:
1) Little-endian int32 (4 bytes) representing the REAPIv2
   DigestFunction.
2) Little-endian int64 (8 bytes) representing the number
   of bytes in the blob.
3) The hash bytes from the digest, length determined by
   the particular DigestFunction.
   (32 for SHA256. 20 for SHA1, 16 for MD5).

Note that we currently only support SHA256, however.

This header is simple to parse, and does not require buffering the
entire blob in memory if you just want the data.

To distinguish blobs with and without this header, we use new
directories for the affected blobs: ac.v2/ instead of ac/ and
similarly for raw/.

We do not use this header to actually verify data yet, and we
still os.File.Sync() after file writes (buchgr#67).

This also includes a slightly refactored version of PR buchgr#123
(load the items from disk concurrently) by @bdittmer.

cache/disk/BUILD.bazel

@@ -4,6 +4,7 @@ go_library(
     name = "go_default_library",
     srcs = [
         "disk.go",
+        "load.go",
         "lru.go",
     ],
     importpath = "github.com/buchgr/bazel-remote/cache/disk",
@@ -29,5 +30,6 @@ go_test(
         "//cache:go_default_library",
         "//cache/http:go_default_library",
         "//utils:go_default_library",
+        "@com_github_bazelbuild_remote_apis//build/bazel/remote/execution/v2:go_default_library",
     ],
 )