Redundant copy cleanup (NFC)

[sfu2]

This PR fixes a few unnecessary copies related to containers.

1. replace push_back with emplace_back to avoid a temporary copy of Datafile
2. iterate unordered map with reference to avoid implicit container copy
3. rewrite tpool::circular_queue::resize to resize the vector in place
   If m_head > m_tail, resize and update m_capacity. Else, shift elements after m_tail to the end of the vector.

[CLAassistant]

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