mem-cache: Fix include directives in the cache related classes

Change-Id: I111b0f662897c43974aadb08da1ed85c7542585c
Reviewed-on: https://gem5-review.googlesource.com/10433
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Remove unused 'using namespace'

Removal of unused/barely used 'using namespace' from C++ files.

Change-Id: I66dc548c04506db2e41180b9ea7ab5abd7d5375a
Reviewed-on: https://gem5-review.googlesource.com/9601
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Deallocate all write-queue entries when sent

This patch removes the write-queue entry tracking previously used for
uncacheable writes. The write-queue entry is now deallocated as soon
as the packet is sent. As a result we also forego the stats for
uncacheable writes. Additionally, there is no longer a need to attach
the write-queue entry to the packet.

mem: Create a separate class for the cache write buffer

This patch breaks out the cache write buffer into a separate class,
without affecting any stats. The goal of the patch is to avoid
encumbering the much-simpler write queue with the complex MSHR
handling. In a follow on patch this simplification allows us to
implement write combining.

The WriteQueue gets its own class, but shares a common ancestor, the
generic Queue, with the MSHRQueue.