Add a multiprocessing lock to index syncing

[dmaljovec]

Fixes #586

Description of changeset:

Add a lock to the code in charge of syncing the index

Test Plan:

Run an instance using more than one gunicorn workers (this is a default, but a minimal example with 2 workers is shown below):

knowledge_repo --repo path/to/repo deploy -w 2

and ensure no more error messages show up that look like:

Process Process-1:1:
Traceback (most recent call last):
  File ".../python3.6/site-packages/knowledge_repo/app/models.py", line 134, in wrapped
    return function(*args, **kwargs)
  File ".../python3.6/site-packages/knowledge_repo/app/index.py", line 141, in update_index
    current_repo.update()
  File ".../python3.6/site-packages/knowledge_repo/repositories/gitrepository.py", line 144, in update
    self.git.branches[branch].checkout()
  File ".../python3.6/site-packages/git/refs/head.py", line 219, in checkout
    self.repo.git.checkout(self, **kwargs)
  File ".../python3.6/site-packages/git/cmd.py", line 542, in <lambda>
    return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
  File ".../python3.6/site-packages/git/cmd.py", line 1005, in _call_process
    return self.execute(call, **exec_kwargs)
  File ".../python3.6/site-packages/git/cmd.py", line 822, in execute
    raise GitCommandError(command, status, stderr_value, stdout_value)
git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
  cmdline: git checkout master
  stderr: 'fatal: Unable to create '.../repo/.git/index.lock': File exists.