Prefer a timestamp from the database

[data-sync-user]

We often defer to the database for time keeping such as when filtering against expiry via CURRENT_TIMESTAMP (diesel::dsl::now).

The Spanner impl also does this, but it also goes one step further and additionally gets itsSyncTimestamp value from the database. It already needs to query the db in lock_for_write before making any writes, so it might as well ask for the timestamp there anyway.

A side effect of this change is it also forces the SyncTimestamp to be None for read methods, which adds an extra check layer to them as they should never require it, not making any writes.

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