Add support for resuming an incomplete sync

[batkinson]

Due to the fact that synchronization of large files can take a good amount of time, the expectation that the entire process completes in one attempt is often unrealistic. To allow for incremental progress in these scenarios, support should be added for resuming an aborted sync and resuming where it left off. This should provide better performance especially in Android, where Sync Adapters can actually be halted if they take too long.

[batkinson]

Essentially, this could be as simple as exposing enough of a public API to detect and work with the raw components of a sync process. Namely:

• Persistent metadata
• Persistent block search results
• Existing partial file

With this, the user should be able to skip metadata download, block search and fetching previously fetched remote content, respectively.