Arcyd: start and stop commands use pids, abdt_fs

Make Arcyd start and stop able to tell if an instance is already
running and potentially kill it by using it's pid.

Make use of the new abdt_fs to simplify disk operations.

Test Plan:
$ ./precommit.sh

start and stop with
$ ./testbed/arcyd/test_shell.sh
Arcyd: refactor, mv initialise to abdt_fs

Move the bulk of the work of 'arcyd init' to the new abdt_fs module.

Test Plan:
$ ./precommit.sh
Arcyd: refactor, extract 'add-phabricator' fs work

Move the work that 'arcyd add-phabricator' does with the filesystem
to the new abdt_fs module.

Test Plan:
$ ./precommit.sh
Arcyd: refactor, extract 'add-repo' fs work

Move the work that 'arcyd add-repo' does with the filesystem
to the new abdt_fs module.

Test Plan:
$ ./precommit.sh
Arcyd: refactor, extract abdt_fs..._create_config

Extract common config creation code to abdt_fs.Accessor._create_config

Test Plan:
$ ./precommit.sh
Arcyd: add-repo: fetch existing special refs

We want to continue the history of the 'landed' and 'abandoned' refs
if we're importing repositories into a new arcyd instance.

Check if the refs exist and if so then fetch them.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

land a branch and then create a new arcyd instance in
$ ./testbed/arcyd/test_shell.sh