mem-cache: Encapsulate CacheBlk's task_id

mem-cache: Encapsulate CacheBlk's task_id

Encapsulate this variable to facilitate polymorphism.

- task_id was renamed to _taskId and was privatized.
- The task id should only be modified at 2 specific moments:
  insertion and invalidation of the block; thus, its setter
  is not public.

Change-Id: If9c49c22117ef5d7f25163ec94bf8b174f221e39
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34956
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>