systemc: self-manage TimeSlot in Scheduler
systemc: self-manage TimeSlot in Scheduler

TimeSlot is new and deleted frequently. Having a recycling memory
manager can help saving the time spent new and delete. Tested and see
about 4% improvement in simulation speed.

Change-Id: I0ab173168336a883b85f768d7fdf07a936a14d69
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34615
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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