_log_level_resets is a map<str, level_reset>, we want to extract the
next expiring level_reset to rearm the _log_level_timer.

previously we would extract the min_element based on the key (the logger
name)

so this sequence of actions
set log-level kafka trace 10
set log-level raft trace 10

only kafka would be expired, because once expired the next run would
pick again kafka, see that there is no expiration, and not rearm the
timer.

(cherry picked from commit bf6bc91)

(cherry picked from commit 0717cd1)

the test shows that this sequence of actions

set log-level admin-api-server trace 10
set log-level raft trace 10

works and both logger are reset after the expiry period

(cherry picked from commit 6593dbe)

previously, this sequence of actions

set log-level raft trace 10
set log-level raft error 0

would result in raft logging at error level for only 10 seconds, despite
the legit request to expire never.

this is because in the code, the second request would not correctly
reset the expire field if a previous name-level_reset was already in the
map used for this operation

(cherry picked from commit d781fb1)

test that
set log-level admin_api_server trace 10
set log-level admin_api_server error 0

never resets the logger to the info level

(cherry picked from commit 84748a8)