Implement a Git LFS link table and basic batch API

Summary:
Ref T7789. This implements:

• A new table to store the <objectHash, filePHID> relationship between Git LFS files and Phabricator file objects.
• A basic response to batch commands, which return actions for a list of files.

Test Plan:
Ran git lfs push origin master, got a little further than previously:

epriestley@orbital ~/dev/scratch/poemslocal $ git lfs push origin master
Git LFS: (2 of 1 files) 174.24 KB / 87.12 KB
Git LFS operation "upload/b7e0aeb82a03d627c6aa5fc1bbfd454b6789d9d9affc8607d40168fa18cf6c69" is not supported by this server.
Git LFS operation "upload/b7e0aeb82a03d627c6aa5fc1bbfd454b6789d9d9affc8607d40168fa18cf6c69" is not supported by this server.

With GIT_TRACE=1, this shows the batch part of the API going through.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D15489