Stream buffer hit handler implementation.

Implemented stream_buffer_hit_handler() to handle prefetching
when the beginning of the stream buffer is used.

Also added some debug flags to print stream buffer status info
to standard error. Also added some counters to count the number
of streams generated during runtime.

Finally, added a bunch of traces to stress test the stream buffer.
These traces were generated with trace_gen.py, a configurable trace
generator for HybridSim.