perf: reduce copying in enforceSwarmPeerLimit()

[tearfur]

A small improvement that should help with #5714.

[ckerr]

Nice! TIL about std::partial_sort_copy()

[tearfur]

I am actually quite looking forward to C++20 so I can just join views of the peers vectors of each torrent and make one single pass of std::partial_sort_copy() in enforceSessionPeerLimit() as well.

Edit: Even more so in here, C++20 views is probably gonna help quite a bit.

transmission/libtransmission/peer-mgr.cc

Lines 2408 to 2428 in 2a9c30a

	for (auto& [socket_address, atom] : swarm->pool)
	{
	if (isPeerCandidate(tor, atom, now))
	{
	candidates.push_back({ getPeerCandidateScore(tor, atom, salter()), tor, &atom });
	}
	}
	}
	// only keep the best `max` candidates
	if (std::size(candidates) > max)
	{
	std::partial_sort(
	std::begin(candidates),
	std::begin(candidates) + max,
	std::end(candidates),
	[](auto const& a, auto const& b) { return a.score < b.score; });
	candidates.resize(max);
	}
	return candidates;