Update massif implementation to xtree

Update massif implementation to xtree
* update massif code so as to replace the massif specific XTree implementation
  by calls to pub_tool_xtree.h
  This improves the massif performance in CPU (a few percents to a factor
   2 gained, depending on the implementation). The memory use is also
   decreased.
  With this patch, massif behaves similarly to the previous implementation.
  The differences are:
     * some massif stats output have changed, as the new implementation
       differs completely from the previous one.
     * when 2 'branches' of the call tree have consumed the same amount
       of  heap, the order in which the branches are output in the massif
       output file can differ between the old and new implementation.
  No other functional difference is expected, waiting for new bugs to
  be discovered.

* Also add support for xtree memory profiling and xtmemory gdbsrv monitor
  command in massif :
  * massif will produce xtree memory profiling according to the options
    --xtree-memory.
  * addition of the xtmemory gdbserver monitor command.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16129