We shouldn't dispatch all outputs right away since they
can be expensive CPU-wise. Instead, rely on DESTROY to
trigger further redispatches.
This also fixes a circular reference bug for the single-output
case that could lead to a leftover script/lei after MUA exit.
I'm not sure how --jobs/-j should work when the actual xsearch
and lei2mail has it's own parallelism ("--jobs=$X,$M"), but
it's better than having thousands of subtasks running.
Fixes: b34a267efff7b831 ("lei up: fix --mua with single output")