job: add a persistence mechanism to local disk

[airwoodix]

Summary

This patch adds the possibility to persist AQTJob handles to local disk. This should help writing more resilient Qiskit scripts.

Details and comments

The default storage is the user cache directory for the package, including its version. Since the storage should be relatively short lived, this is not a serious limitation for the end user and makes sure we can modify the storage model in backwards-incomaptible ways in future releases.

The serialization of Qiskit circuits is provided by qiskit.qpy. This is in principle only required for offline simulator jobs, but users may rely on having access to the job details (including the exact QuantumCircuits) from the job handle. This data cannot be faithfully reconstructed from the data served by the cloud portal.

The functionality has two caveats when handling jobs that run on offline simulators:

1. the state of the offline simulators cannot be easily persisted, and would lead to an overly complex persistence format. Therefore, offline simulator jobs are re-submitted when restored, which leads to a new job_id and varying results for each call to AQTJob.restore. This surprising behavior is clearly indicated in the API reference and the user guide.
2. for simplicity, restoring an offline simulator job with noise model enabled discards the noise model. The roots of this problem are the same as point 1, but will be worked around in a follow-up patch.

Breaking change: as part of refactoring the resource initializers, renamed OfflineSimulatorResource.noisy to OfflineSimulatorResource.with_noise_model. This addresses #72 (comment).