Morango

- Fixes issue handling database counters which caused repeat syncing of unchanged data

- Fixes slow performance due to excessive use of `sleep`

- Fix to set counters on `TransferSession` *after* serialization
- Fix capabilities request header as it should be prefixed with `HTTP`
- Fix issues with flow of transfer operations
- Logging and error handling improvements

- 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