master: remove weaken on storable::read

6d62e6b0782ecfa0f4e0c2bec14434d675734962 introduced a weaken_config() sub, used
in munin_readconfig_storable().  It should not be needed, and some profiling
showed that it is a *huge* cpu eater.
master: remove auto_weaken()

Every weaken() should be done on construction time. Fixing it afterwards is
wrong, and might be dangerous. Let bug leaks happen and be fixed !
contrib: add a test for memory leaks
contrib: complete test with Storable dump/load

Test if Storable also saves the "weak" state.

Hint: it does.