Provide tool to filter drmemtrace offline traces

It will be useful to have a tool that can filter out entries that are not needed in an offline drmemtrace trace, after it has been processed by raw2trace.

I imagine this can be implemented as a new drmemtrace analyzer that processes each thread's trace separately. The current trace reader returns a memref_t and hides some entries from the analyzer. We'd need to support reading raw trace_entry_t entries.