Adding buffer pool to JitChannelQueue.
Adding buffer pool to JitChannelQueue.

Rather than hit the heap for every message, we now maintain a pool of buffers (lazily-created) for enqueued messages.

Minor improvement: 160k-tick SerialProcRuntimeTest.CarriesState goes from 59s to 50s, but I suspect that a network with a more complicated communication pattern would show greater improvement.

PiperOrigin-RevId: 339113002

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