Create a spec for crash recovery and retry during blobs sync

Roadmap activity 2.3. Implement a minimally functional transfer retry strategy

The following are the items in the activity.

• unified download/upload manager (blobmanager)
• Minimal crash recovery for the blobs network transfer manager
• implement a retry strategy (for interrupted network)
• ability to save transfer metadata locally
• detect incomplete upload/download
• properly notify a verification failure (should mark the downloaded blob as unusable)

Definition of Done

1. Document clearly (on this issue) what are the cases that we want to prevent (ie, all the possible failure modes of the blob manager).
2. Document what are the possibilities to provide retries and resumable transfers in the client.
3. Choose a subset of features to implement in a first phase.
4. Create issues for all of the suggested features, prioritized by roadmap milestone.