zed.d/history_event-zfs-list-cacher.sh.in: parallelise, simplify

This:

(a) improves the error log message,
(b) locks per pool instead of globally,
(c) locks the actual output file instead of /var/lock/zfs-list,
    which would otherwise linger there forever (well, still will,
    but you can remove it and it won't come back), and
(d) preserves attributes of the output file
    instead of reverting them to 0:0 644

It is imperative that the previous commit
("zed-functions.sh: zed_lock(): don't truncate lock")
be included in any series that contains this one

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12042