reduce false sharing in LogsDBStats

[izabera]

these fields are all atomic so the accesses are safe, but different threads end up stepping on each other's toes when accessing different fields

this adds padding and groups together the fields accessed at the same time

• the first group is the hot path, which are updated together in the main event loop
• then there's the request/response management group
• then the reader group
• then the catchup group
• and finally there's a misc metadata group, which are touched independently at different times

[bitonic]

I personally wouldn't bother -- this is going to be totally immaterial performance-wise, and obviously prone to breaking if the code changes, and so it is at a very high risk of just remaining in the code as is without anybody caring about whether the groupings still make sense.

[mcrnic]

I don't mind but I personally would not bother. LogsDB is single threaded at the moment. The only other thing accessing these stats is stat exporter which kicks off once a minute.

[bitonic]

BTW, If we were to merge this, I'd definitely comment the alignas. As a compromise, I think merging without the alignas is harmless.