Fix the timing of read and write events under kqueue (#9416)
Normally we execute the read event first and then the write event. When the barrier is set, we will do it reverse. However, under `kqueue`, if an `fd` has both read and write events, reading the event using `kevent` will generate two events, which will result in uncontrolled read and write timing. This also means that the guarantees of AOF `appendfsync` = `always` are not met on MacOS without this fix. The main change to this pr is to cache the events already obtained when reading them, so that if the same `fd` occurs again, only the mask in the cache is updated, rather than a new event is generated. This was exposed by the following test failure on MacOS: ``` *** [err]: AOF fsync always barrier issue in tests/integration/aof.tcl Expected 544 != 544 (context: type eval line 26 cmd {assert {$size1 != $size2}} proc ::test) ``` (cherry picked from commit 306a5ccd)
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