Utilize on conflict update in SpannerDb::update_user_collection_quotas

[data-sync-user]

The final DML of this method checks if there’s a user_collections entry when there’s no bsos. This could be simplified into a single insert on conflict vs the current, 3 separate sql calls.

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[data-sync-user]

Philip Jenvey commented from Jira:

I think another small improvement here would be to make this method work a lot more like the other implementations, with a clearer separation between whether quota’s enabled or not:

https://github.com/mozilla-services/syncstorage-rs/pull/2303#pullrequestreview-4283922084

As I mentioned there, there’s no need to query bsos at all here when quota is disabled. We should continue not writing to the quota columns in the disabled version (note that the other impls just hardcode writes of 0 to those columns), as it needlessly adds to the Spanner mutation count.