Sync method to propagate deletions in batch locally.

-- Resolves: #8961

-- Related: #8961

As defined in #8970, this table and the new module will ease adding sync
features such as priority queues and streaming.

--Resolves: #8970

- add a MaximumRetriesError exception to encapsulate other exceptions.
- record the pending status before trying to download
- modify update_sync_status to insert or update
- modify retry tests to check number of retries
- add a test for download retry limit

Notify, log something meaninful and retry at most 3 times before marking
the download as unusable (FAILED_DOWNLOAD).

-- Related: #8825

Added retry to upload and modified retry implementation to comply with
discussed spec.
According to it, we should wait between retries, something like 1s, 10s,
.. up to 1 minute.

-- Resolves: #8822

-- Related: #8822

Instead of querying the server, fetch_missing and send_missing now uses
the PENDING_DOWNLOAD and PENDING_UPLOAD statuses to guide itself on what
to do. This allows the sync mechanism to control when/how to query data
from server and reuse the query data during the sync.

-- Related: #8822

We were comparing the raw content of preambles. This commit adds a way
to compare excluding time so comparisons don't suffer from false
negatives caused by time deltas.

-- Resolves: #8920