drx_buf buffer pointer update improvements

I don't think I'll have time to get to this now, but I'm documenting it here for later (I'm excited for this).

Currently, the existing code for `drx_buf_insert_update_buf_ptr()` surfaces an implementation detail whereby the user must be weary of "torn writes", documented [here][drx_buf-link]. This issue proposes making the current buffer update operation act more like a logical commit operation, by eliminating the concept of torn writes.

This will likely require *slight* changes to `drx_buf_insert_buf_memcpy()` as well as to the buffer pointer reset logic.

Does this sound good?

[drx_buf-link]: http://dynamorio.org/docs/page_drx.html#sec_drx_buf_trace