- 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

Tests entrypoint was in a testing/ subfolder in the root of the
repository. This was made mainly because we had some common files for
tests and we didn't want to ship them (files in testing/test_soledad,
which is itself a python package. This sometimes causes errors when
loading tests (it seems setuptools is confused with having one python
package in a subdirectory of another).

This commit moves the tests entrypoint to the root of the repository.

Closes: #8952