Use pointer instead of iterator for thread safe unordered_map usage

[ypatia]

While reading remote global order writes code, chasing a corruption issue, I spotted that we were accessing an iterator of an unordered_map without synchronization which is not thread safe (afaik concurrent emplace() calls for other URIs may trigger an unordered_map rehash which invalidates iterators). I replaced the iterator with a pointer to the data instead to fix this

Relates to CLOUD-3368

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TYPE: BUG
DESC: Use pointer instead of iterator for thread safe unordered_map usage

[teo-tsirpanis]

We are under a reader lock; how is this unsafe? Writes aren't suppose to happen during that code path.

[ypatia]

We are under a reader lock; how is this unsafe? Writes aren't suppose to happen during that code path.

There is a unique_rl.unlock(); in both branches of the if statement, so we are not in a reader lock.

[teo-tsirpanis]

Approved per offline explanation.