Fix write bug in tkldb and add spamfilter::action stop.

Fix write bug in tkldb and add spamfilter::action stop.
The spamfilter::action stop ill prevent processing other spamfilters.
This would normally be a bit unusual, and potentially dangerous when you
do exclude things this way, but can be useful in some circumstances.

Stopping only affects the same type of spamfilters (general or central
spamfilters), so they don't interfere.

The tkldb write DB bug had to do with that it was processing
central spamfilters, which should be skipped just like config
based spamfilters were already skipped.