Discard empty command histories before discarding LRU non-empty ones

[serd2011]

When selecting candidate history, priority is given to histories with empty commands list.

I think lazily calling s_Allocate will require a ton of rewriting. Also I think it is not an unexpended behavior that buffer is allocated when an application connects.

Closes #13571

[lhecker]

I get the feeling that there's some way to write this without the need for 3 OR'd conditions... Something like this (pseudocode)?

const auto beg = s_historyLists.crbegin();
const auto end = s_historyLists.crend();

auto candidate = std::find_if(beg, end, [](const auto& it) {
    return WI_IsFlagClear(it.Flags, CLE_ALLOCATED);
});

const auto optimalCandidate = std::find_if(candidate, end, [](const auto& it) {
    return WI_IsFlagClear(it.Flags, CLE_ALLOCATED) && it._commands.empty();
});
if (optimalCandidate != end)
{
    candidate = optimalCandidate;
}

if (candidate != end)
{
    s_historyLists.erase(std::next(it).base());
}

...but I'm not convinced that this is an improvement.

[DHowett]

Thanks for implementing this! I reworded it slightly so that GitHub didn't "wrap" the title once it committed it. 😄

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[ghost]

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