Asynchronous transfers with a task queue and Nearline support

[james-s-willis]

@jrs65 @ketiltrout I've added the nearline test, so this is ready to review now

[anjakefala]

From a conversation with James.

This PR has the following goals:

• To add an API with which to make file copy requests from nearline tapes. Part of https://github.com/chime-experiment/Pipeline/issues/104 The following documentation was referenced for using lfs to query tape storage: https://docs.computecanada.ca/wiki/Using_nearline_storage
• In order to use nearline, calls now need to be async (nearline has much longer waits than alpenhorn has been previously accustomed to). To accomodate this a Task Queue was added, with a threadpool, whose tasks will then be processed.
• To add tests for the new API. tests/lfs contains a mock for lfs.
• To begin bringing over fixes that have been added to alpenhorn 1, but have not been ported to alpenhorn 2, yet (e.g. https://github.com/chime-experiment/alpenhorn/pull/19).

task.py contains the new Task logic. update.py has been then factored out, and can now use add_task(TaskClass) for every type of Task type.

The build is currently red because some tests check for help output, and one Python package has been updated since the code was written, changing the output. It still needs to be decided whether to fix the version of the Python package or update the tests.

[ketiltrout]

• To begin bringing over fixes that have been added to alpenhorn 1, but have not been ported to alpenhorn 2

Unless there's specific things needed to support the task queue, I would do this part in a separate PR.

[ketiltrout]

Whilst the functional parts of James's work is still present, I've largely rewritten the task/queue framework.

I'll going to create a new PR for that.