Increase segment size of UMPMCQueue to reduce allocations
Increase segment size of UMPMCQueue to reduce allocations

Summary:
UMPMCQueue is used as a retrieval queue on our request critical path.
We are increasing the segment size to reduce the allocations. (256B -> 1KB)
The max potential mem cost per queue is 40KB (we have 5 queues)

Reviewed By: yfeldblum

Differential Revision: D33857254

fbshipit-source-id: f857b7712123bcf6de2e8b3e28a3d74cf0e22b1d

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