p/debug: adding a huge multigraph plugin

Ganneff on IRC reported some performance issue with munin-update and huge
multigraph plugins. As it can happen on LUN-flooded hosts, with the stock
diskstats plugin, it has to be fixed.

Therefore this plugin will serve as a test one, to emulate those gigantic
plugins outputs.
p/debug: speedup multigraph_huge by precomputing
Merge pull request #253 from steveschnepp/multigraph_huge

Multigraph huge