Adapt PoolState to deadpool

[data-sync-user]

The PoolState type returned by DbPool::state is based off r2d2::State. After switching to diesel-async we’ll have switched to deadpool for all db pools' implementations.

We might want to consider renaming state → status (after deadpool::Status) and adapting the struct field names to deadpool’s names/types as well.

Putting this in the cleanup epic for now but we might move it elsewhere (to do later).

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