Bugfixes/improvements to mt_tbs.py

Bugfix: forced addr in addTransaction to use c_ulonglong(). Previously,
addr was getting cut off at 32-bits for large addresses.

Improvement: Added THREAD_PENDING_MAX to track how many outstanding
accesses are allowed per thread. When the number of outstanding
accesses is at its max, the throttle_cycles is incremented instead of
trace_cycles. This was also necessary to get decent performance for many
cases (otherwise, the HybridSim pending queue would get massive and slow
things down to a halt).