mem: Fill the new packet ID fields with master IDs when tracing packets.

This will let somebody consuming the memory packet trace make sense out of
the master IDs passed along with individual accesses.

Change-Id: I621d915f218728066ce95e6fc81f36d14ae7e597
Reviewed-on: https://gem5-review.googlesource.com/4800
Reviewed-by: Rahul Thakur <rjthakur@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Trace the request master ID in the MemTraceProbe.

There's a spot for it in the packet trace protobuf, so we should fill it
with something.

Change-Id: I784feb3f668e1b20d67b6ef98d012bcf59b7bd40
Reviewed-on: https://soc-sim-internal-review.googlesource.com/3483
Reviewed-by: Rahul Thakur <rjthakur@google.com>
Reviewed-on: https://gem5-review.googlesource.com/4781
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Add Program Counter to MemTraceProbe

mem: Add PacketInfo to be used for packet probe points

This patch fixes a use-after-delete issue in the packet probe points
by adding a PacketInfo struct to retain the key fields before passing
the packet onwards. We want to probe the packet after it is
successfully sent, but by that time the fields may be modified, and
the packet may even be deleted.

Amazingly enough the issue has gone undetected for months, and only
recently popped up in our regressions.

mem: Move trace functionality from the CommMonitor to a probe

This changeset moves the access trace functionality from the
CommMonitor into a separate probe. The probe can be hooked up to any
component that exports probe points of the type ProbePoints::Packet.

This patch moves the dependency on Google's Protocol Buffers library
from the CommMonitor to the MemTraceProbe, which means that the
CommMonitor (including stack distance profiling) no long depends on
it.