Simplify (?) arc upload code and make it more future-oriented

Summary: See discussion in D4968. This makes us run `file.uploadhash` calls in parallel, to slightly improve performance.

Test Plan:
Created a binary change in a commit:

  $ head -c65535 /dev/urandom > a_random_file
  $ git add .
  $ git commit -m .
  [master 32e4c0a] .
   1 files changed, 0 insertions(+), 0 deletions(-)
   create mode 100644 a_random_file

Verified `arc diff` called `conduit.query`, then `file.uploadhash` (which we expect to fail; the file is new), then `file.upload`:

  $ arc diff HEAD^ --only --trace
  ...
  Uploading 1 files...
  >>> [15] <conduit> conduit.query() <bytes = 157>
  >>> [16] <http> http://local.aphront.com/api/conduit.query
  <<< [16] <http> 111,500 us
  <<< [15] <conduit> 112,169 us
  >>> [17] <conduit> file.uploadhash() <bytes = 254>
  >>> [18] <http> http://local.aphront.com/api/file.uploadhash
  <<< [18] <http> 85,386 us
  <<< [17] <conduit> 85,798 us
  >>> [19] <conduit> file.upload() <bytes = 97009>
  >>> [20] <http> http://local.aphront.com/api/file.upload
  <<< [20] <http> 144,098 us
  <<< [19] <conduit> 144,550 us
  Uploaded 'a_random_file' (new).
  Upload complete.
  ...
  Created a new Differential diff:
          Diff URI: http://local.aphront.com:8080/differential/diff/202/

  Included changes:
    A (bin) a_random_file

Created the same diff again, verified that we skip the data transfer this time:

  $ arc diff HEAD^ --only --trace
  ...
  Uploading 1 files...
  >>> [15] <conduit> conduit.query() <bytes = 157>
  >>> [16] <http> http://local.aphront.com/api/conduit.query
  <<< [16] <http> 112,717 us
  <<< [15] <conduit> 113,374 us
  >>> [17] <conduit> file.uploadhash() <bytes = 254>
  >>> [18] <http> http://local.aphront.com/api/file.uploadhash
  <<< [18] <http> 95,545 us
  <<< [17] <conduit> 95,921 us
  Uploaded 'a_random_file' (new).
  Upload complete.
  ...
  Created a new Differential diff:
          Diff URI: http://local.aphront.com:8080/differential/diff/203/

  Included changes:
    A (bin) a_random_file

Created a new commit which adds a new file and updates the exiting file:

  $ head -c65535 /dev/urandom > another_file
  $ head -c65535 /dev/urandom >> a_random_file
  $ git add .
  $ git commit -m .
  [master 28f4fbd] .
   2 files changed, 0 insertions(+), 0 deletions(-)
   create mode 100644 another_file

Verified `arc diff` transfers one file by hash (old version of the first file) and two by data (new first file, new second file):

  $ arc diff HEAD^ --only --trace
  ...
  Uploading 3 files...
  >>> [19] <conduit> conduit.query() <bytes = 157>
  >>> [20] <http> http://local.aphront.com/api/conduit.query
  <<< [20] <http> 114,825 us
  <<< [19] <conduit> 115,517 us
  >>> [21] <conduit> file.uploadhash() <bytes = 254>
  >>> [22] <http> http://local.aphront.com/api/file.uploadhash
  >>> [23] <conduit> file.uploadhash() <bytes = 254>
  >>> [24] <http> http://local.aphront.com/api/file.uploadhash
  >>> [25] <conduit> file.uploadhash() <bytes = 253>
  >>> [26] <http> http://local.aphront.com/api/file.uploadhash
  <<< [24] <http> 104,310 us
  <<< [26] <http> 105,211 us
  <<< [25] <conduit> 105,587 us
  <<< [22] <http> 112,631 us
  <<< [25] <conduit> 111,437 us
  Uploaded 'a_random_file' (old).
  <<< [25] <conduit> 111,678 us
  >>> [27] <conduit> file.upload() <bytes = 193912>
  >>> [28] <http> http://local.aphront.com/api/file.upload
  >>> [29] <conduit> file.upload() <bytes = 97379>
  >>> [30] <http> http://local.aphront.com/api/file.upload
  <<< [28] <http> 153,126 us
  <<< [29] <conduit> 161,180 us
  Uploaded 'a_random_file' (new).
  <<< [30] <http> 678,739 us
  <<< [29] <conduit> 679,121 us
  Uploaded 'another_file' (new).
  Upload complete.
  ...
  Created a new Differential diff:
          Diff URI: http://local.aphront.com:8080/differential/diff/204/

  Included changes:
    M (bin) a_random_file
    A (bin) another_file

Ran the same command again, verified three uploads by hash:

  $ arc diff HEAD^ --only --trace
  ...
  Uploading 3 files...
  >>> [19] <conduit> conduit.query() <bytes = 157>
  >>> [20] <http> http://local.aphront.com/api/conduit.query
  <<< [20] <http> 117,058 us
  <<< [19] <conduit> 117,792 us
  >>> [21] <conduit> file.uploadhash() <bytes = 254>
  >>> [22] <http> http://local.aphront.com/api/file.uploadhash
  >>> [23] <conduit> file.uploadhash() <bytes = 254>
  >>> [24] <http> http://local.aphront.com/api/file.uploadhash
  >>> [25] <conduit> file.uploadhash() <bytes = 253>
  >>> [26] <http> http://local.aphront.com/api/file.uploadhash
  <<< [22] <http> 103,373 us
  <<< [24] <http> 105,418 us
  <<< [26] <http> 105,251 us
  <<< [25] <conduit> 105,604 us
  Uploaded 'a_random_file' (old).
  <<< [25] <conduit> 105,844 us
  Uploaded 'a_random_file' (new).
  <<< [25] <conduit> 106,053 us
  Uploaded 'another_file' (new).
  Upload complete.
  ...
  Created a new Differential diff:
          Diff URI: http://local.aphront.com:8080/differential/diff/205/

  Included changes:
    M (bin) a_random_file
    A (bin) another_file
  $

Reviewers: kwadwon

Reviewed By: kwadwon

CC: aran

Maniphest Tasks: T2456

Differential Revision: https://secure.phabricator.com/D4993