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.
Commit edba2e97 renamed pageserver/README to pageserver/README.md, but
forgot to update links to it. Fix.
Rename libs/postgres_ffi/README and safekeeper/README files to also
have the the .md extension, so that github can render them nicely.
Quote ascii-diagram in safekeeper/README.md so that it renders
correctly.
