blk/KernelDevice: Unify discard thread management

blk/KernelDevice: Unify discard thread management

Instead of having _discard_start() and _discard_stop() partially or
completely duplicate functionality in handle_conf_change(), have a
single _discard_update_threads() that can handle all three. Loops are
tidied slightly, the unnecessary target_discard_threads class variable
has been removed, and now handle_conf_change() will respect
support_discard.

Signed-off-by: Joshua Baergen <jbaergen@digitalocean.com>