Some improvements for malloc/free intensive programs, inspired by

Some improvements for malloc/free intensive programs, inspired by
performance studies by Christoph Bartoschek:

* Increase the number of freelists per arena from 18 to 112, so as
  to (drastically) cut down on the amount of freelist searching that
  happens.

* Increase the size of the client and tool arenas, so as to reduce
  the cost of finding arenas during freeing.  This is a kludge; a
  better solution would be to use binary search on superblocks, as
  Christoph's patches do.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@6770