Don't allocate in BTreeMap when only one postbox is needed
#819
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We keep track of `MsgBlkChain`s in a `BTreeMap` today. Sadly this isn't a type which we can preallocate a desired capacity in and store per-CPU. As a result, for flamegraphs in the Rx pathway we can see a moderately chunky call into `kmem_alloc` within `Postbox::post` for chains which have one recipient. This PR special-cases the single-recipient case so that we hold onto one chain and key on the heap until needed.
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We keep track of
MsgBlkChains in aBTreeMaptoday. Sadly this isn't a type which we can preallocate a desired capacity in and store per-CPU. As a result, for flamegraphs in the Rx pathway we can see a moderately chunky call intokmem_allocwithinPostbox::postfor chains which have one recipient.This PR special-cases the single-recipient case so that we hold onto one chain and key on the heap until needed.