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9ede38b6c4
A new `get_lsn_by_timestamp` command is added to the libpq page service API. An extra timestamp field is now stored in an extra field after each Clog page. It is the timestamp of the latest commit, among all the transactions on the Clog page. To find the overall latest commit, we need to scan all Clog pages, but this isn't a very frequent operation so that's not too bad. To find the LSN that corresponds to a timestamp, we perform a binary search. The binary search starts with min = last LSN when GC ran, and max = latest LSN on the timeline. On each iteration of the search we check if there are any commits with a higher-than-requested timestamp at that LSN. Implements github issue 1361.