We have a few std::list usages where the most common case is a single element. In the most common case, we're heap-allocating and freeing a single element, which is wasteful. Similar to absl::InlinedVector, we can make a linked-list-like container where the first node is allocated inline with the container, making the first node free. All future elements will just default to heap-allocation.

This would benefit perf in two hot code paths:

• OperationCpuTimers
• TransactionResources
• WiredTigerSessionCache

Both of these linked lists are most commonly of size 1 and in the hot path of every operation. I observed about a 500ns speedup in the TransactionResources path, so I would expect about a 1 microsecond speedup by changing both, assuming we can't find others.