DEBUG set-active-expire added.

We need the ability to disable the activeExpireCycle() (active
expired key collection) call for testing purposes.
Test: verify that lazy-expire works.
Better DEBUG error message when num of arguments is wrong.
Test: regression test for issue #1026.
EXPIRE should not resurrect keys. Issue #1026.
Merge remote-tracking branch 'antirez/2.6' into 2.6