Make transfer rate metric per-processor in scheduler

[riteshbhirud]

The transfer rate metric in the scheduler was a single global Ref{UInt64},
unlike other metrics like worker_time_pressure which are per-processor.
This expands transfer_rate to worker_transfer_rate::Dict{Int,Dict{Processor,UInt64}}
following the same pattern, so each processor accumulates its own observed transfer rate
rather than averaging across all processors.

[jpsamaroo]

Thanks so much!