Morango

- Track the `TransferSession` that last modified a `Store` record
- Add state attributes to `TransferSession` for persisting its stage and status during a sync
- Update the timestamp of the last activity for a `SyncSession` and `TransferSession` during a sync
- Add support for resuming a sync
- Add support for configuring custom handling of transfer operations
- Add support for handling transfer operations asynchronously

- Add management command for garbage collection of buffer data
- Speed up instance ID calculation by skipping hostname check

- Allow MAC address to be overridden by envvar for instance_id calculation

- Don't die on session deletion when transfersession.records_total is None

- Cache the instance ID on app load, to avoid database lockup issues

- Split up `SyncClient` and fix bandwidth tracking (https://github.com/learningequality/morango/pull/85)