Adds the concept of "marker entries" to drcachesim's traces, which can be used to store metadata about a location in a trace.
Uses DR's new kernel xfer event to insert trace marker entries on kernel-mediated control transfers: signal handlers, signal returns, Windows APCs, exceptions, callbacks, etc.
To help with testing, adds a new tool basic_counts which counts the number of each trace entry type per thread in the trace.
Adds new online and offline tests that run the basic_counts tool it on the common.decode-bad test binary.
Fixes #2708 (closed)