fix 369468 Remove quadratic metapool alg. using VG_(HT_remove_at_Iter)(VgHashTable...

fix 369468 Remove quadratic metapool alg. using VG_(HT_remove_at_Iter)(VgHashTable *table)

Based on a patch from Ruurd Beerstra
but reworked VG_(HT_remove_at_Iter) so that
the function is implemented without touching the rest of m_hashtable.c
to ensure no performance impact on other hash table usages.

Testing with
for f in 1 2 3 4 5 6 7 8 9; do echo $f;  time ./vg-in-place -q ./memcheck/tests/leak-autofreepool 2 $(expr $f \* 100000); done|&grep user

With the patch :
user	0m0.524s
user	0m0.660s
user	0m0.784s
user	0m0.916s
user	0m1.064s
user	0m1.192s
user	0m1.316s
user	0m1.496s
user	0m1.632s

Without the patch, the same gives:
user	0m4.464s
user	0m16.776s
user	0m24.472s
user	1m5.544s
user	1m21.168s
user	1m40.500s
user	1m54.884s
user	4m58.308s
user	5m34.060s

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16041