mem-cache: Encapsulate CacheBlk's srcRequestorId

mem-cache: Encapsulate CacheBlk's srcRequestorId

Encapsulate this variable to facilitate polymorphism.

- requestorId was renamed to _requestorId and was privatized.
- The requestor ID should only be modified on insertion and
  invalidation; thus, its setter is not public.

Change-Id: I5bab21a6c21e9d912fb5194bb44ff785d44999f4
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34959
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>