Closes: #9000

Adds the ability to have document that wont be synced. This enables
applications to use soledad to store temporary blobs that should be
discarded later instead of unnecessarily keeping the sync loop busy.

-- Resolves: #8819

This commit creates a PENDING_DELETE sync status which can be used to
keep track of whats deleted locally in order to propagate to server
later.

-- Related: #8961

--Resolves: #8848

Retry limit was originally specified in #8825 as a protection mechanism,
but #8822 (retry) doesn't specify a retry limit. In fact, blobs is
supposed to retry until transfer is complete using timed delays between
attempts, but never giving up.

-- Related: #8822
-- Related: #8825

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

--Resolves: #8970

So we can have manager, sync, sql and errors in its own places.

--Related: #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

The way in that concurrency limit was being enforced was such that
transfer attempts were being spawned in groups of 3, and all of them had
to finish before a new group could be spawned. This modification allows
for use of maximum concurrency level at all times.