sim: Add hooks to implement event reference counting

We currently only support deleting an event if it is triggered and not
re-scheduled. This is fine for most native code. However, there are
cases where Python needs to count references to make sure that the
Python object stays live while the native object is live.

Generalise the mechanism used to implement by adding reference
counting hooks to the event base class:

* Event::acquire() / Event::acquireImpl()
* Event::release() / Event::releaseImpl()

These calls can be used to implement both reference counting and the
existing AutoDelete functionality. The default implementation in Event
maintains backwards compatibility with the existing AutoDelete feature
by ignoring acquireImpl() and deleting the event on releaseImpl() if
it isn't scheduled anymore.

Since AutoDelete functionality is no longer the only way events can be
managed, this change introduces the new Managed flag. This flag
activates automatic memory management. The acquireImpl()/releaseImpl()
methods are only called from acquire()/release() it is set. To
maintain backwards compatibility, AutoDelete is used as an alias for
Managed.

Change-Id: I5637984c906a9d44c22780712cf1c521b8297149
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3221
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

sim: simulate with multiple threads and event queues
This patch adds support for simulating with multiple threads, each of
which operates on an event queue. Each sim object specifies which eventq
is would like to be on. A custom barrier implementation is being added
using which eventqs synchronize.

The patch was tested in two different configurations:
1. ruby_network_test.py: in this simulation L1 cache controllers receive
requests from the cpu. The requests are replied to immediately without
any communication taking place with any other level.
2. twosys-tsunami-simple-atomic: this configuration simulates a client-server
system which are connected by an ethernet link.

We still lack the ability to communicate using message buffers or ports. But
other things like simulation start and end, synchronizing after every quantum
are working.

Committed by: Nilay Vaish

sim: have a curTick per eventq
This patch adds a _curTick variable to an eventq. This variable is updated
whenever an event is serviced in function serviceOne(), or all events upto
a particular time are processed in function serviceEvents(). This change
helps when there are eventqs that do not make use of curTick for scheduling
events.