Decrease helgrind memory use for applications allocating many blocks

Use a pool allocator for the MallocMeta struct that helgrind maintains
for each client allocated block.

For perf/heap on x86, this decreases the max amount of mmap-ed memory
for the core area from 56Mb to 36Mb.
On amd64, decreases from 104Mb to 62Mb.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13744 a5019735-40e9-0310-863c-91ae7b9d1cf9