Summary: Instead of hard coding the max deferred readers allowed to be 64, statically allocate large enough slots and pick max deferred readers allowed dynamically based on the platform running the service. Specifically, we set the `maxDeferredReaders = 2 * nextPowTwo(numCPU)`, which four times the number of physical cores, to allow faster reads. We are effectively giving each HW thread two slots.

Reviewed By: yfeldblum

Differential Revision: D22407478

fbshipit-source-id: 4001cf96dc502e00f00a27d57c63ba0028a52671