When performing a write, we should update the view update backlog
on the shard where the mutation is actually applied. Instead,
currently we only update it on the shard that initially received
the write request (which didn't change at all) and as a result,
the backlog on the correct shard and the aggregated max view update
backlog are not updated at all.
This patch enables updating the backlog on the correct shard. The
update is now performed just after the view generation and propagation
finishes, so that all backlog increases are noted and the backlog is
ready to be used in the write response.
Additionally, after this patch, we no longer (falsely) assume that
the backlog is modified on the same shard as where we later read it
to attach to a response. However, we still compare the aggregated
backlog from all shards and the backlog from the shard retrieving
the max, as with a shard-aware driver, it's likely the exact shard
whose backlog changed.
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