* les: move execqueue into utilities package
execqueue is a util for executing queued functions
in a serial order which is used by both les server
and les client. Move it to common package.
* les: move randselect to utilities package
weighted_random_selector is a helpful tool for randomly select
items maintained in a set but based on the item weight.
It's used anywhere is LES package, mainly by les client but will
be used in les server with very high chance. So move it into a
common package as the second step for les separation.
* les: rename to utils
execQueue used an atomic counter to track whether the queue had been
closed, but the checking the counter didn't happen because the queue was
blocked on its channel.
Fix it by using a condition variable instead of sync/atomic. I tried an
implementation based on channels first, but it was hard to make it
reliable.
quit now waits for the queue loop to exit.
