Run "git annex sync" asynchronously

The sync command involves both pushing and fetching from remotes.
Define git annex process calls

Calls to git annex are now through `magit-annex-run' and
`magit-annex-run-async' instead of calling `magit-run-git' and
`magit-run-git-async' directly. `magit-annex-standard-options' are added
to all git annex calls.